Considering the challenge from SABBATH KEEPERS on which day we are commanded to worship on, the answer is found in the clear teaching in the New Testament on what day we are allowed to worship God.

The word for Worship in the Strong's Concordance in  Greek is proskuneo #4352); from 4314 and a derivative of 2965 (meaning to kiss, like a dog licking his master's hand); to fawn or crouch to, i.e. (literally or figuratively) prostrate oneself in homage (do reverence to, adore): WORSHIP. The act of paying honor to a deity; religious reverence and homage. The rendering of the following Heb. and Gk. words: (from New Unger's Bible Dictionary)

The worship of God is nowhere defined in Scripture as one certain thing. It can be many things; to praise; to sing from the heart in praise and thanksgiving. It can be regarded as the direct acknowledgement to God, of His nature, attributes, to submit to his ways and claims, or by how one lives their life. In Acts 17:25 therapeuo, “to serve, do service to” is rendered “is worshipped.”

WORSHIP (VERB AND NOUN), WORSHIPING

1. proskuneo #4352, “to make obeisance, do reverence to” (from pros, “towards,” and kuneo, “to kiss”), is the most frequent word rendered “to worship.” It is used of an act of homage or reverence (a) to God, e. g., <att. 4:10; John 4:21-24; 1 Cor. 14:25; Rev. 4:10; 5:14; 7:11; 11:16; 19:10 (2nd part) and 22:9; (b) to Christ, e. g., Matt. 2:2,8, 11; 8:2; 9:18; 14:33; 15:25; 20:20; 28:9,17; John 9:38; Heb. 1:6>, in a quotation from the Sept. of Deut. 32:43, referring to Christ's second advent; “At the word `worship' in Matt. 2:2, etc., add the marginal note `The Greek word denotes an act of reverence, …

2. sebomai 4576, “to revere,” stressing the feeling of awe or devotion, is used of “worship” (a) to God, Matt. 15:9; Mark 7:7; Acts 16:14; 18:7,133. sebazomai ^4573^, akin to No. 2, “to honor religiously,” is used in Rom. 1:25 4. latreuo ^3000^, “to serve, to render religious service or homage,” is translated “to worship” in< Phil. 3:3>, “(who) worship (by the Spirit of God),” RV, KJV, “(which) worship (God in the spirit)”; the RV renders it “to serve” (for KJV, “to worship”) in< Acts 7:42; 24:14>; KJV and RV, “(the) worshipers” in Heb. 10:2, present participle, lit., “(the ones) worshiping.” See SERVE. 5. eusebeo ^2151^, “to act piously towards,” is translated “ye worship” in Acts 17:23.

The first time the day of worship was given to Israel (it became the Feast of Unleaven Bread). Exod..12:14 “So this day shall be to you a memorial; and you shall keep it as a feast to the LORD throughout your generations. You shall keep it as a feast by an everlasting ordinance. 15 'Seven days you shall eat unleavened bread. On the first day you shall remove leaven from your houses. For whoever eats leavened bread from the first day until the seventh day, that person shall be cut off from Israel. 16 'On the first day there shall be a holy convocation, and on the seventh day there shall be a holy convocation for you. No manner of work shall be done on them; but that which everyone must eat-- that only may be prepared by you.17 `So you shall observe the Feast of Unleavened Bread, for on this same day I will have brought your armies out of the land of Egypt. Therefore you shall observe this day throughout your generations as an everlasting ordinance.”

Exodus 34:4: “So he cut two tablets of stone like the first ones. Then Moses rose early in the morning and went up Mount Sinai, as the LORD had commanded him; and he took in his hand the two tablets of stone.V.8 So Moses made haste and bowed his head toward the earth, and worshiped.”

Was this the Sabbath Day Moses worshiped on? Does God allow us to worship Him any day and time we choose or only on the day and time required in the Old Testament covenant that was given specifically to Israel.

WHO CHANGED THE SABBATH?

Lev. 23:28-32: “And you shall do no work on that same day, for it is the Day of Atonement, to make atonement for you before the LORD your God.  “For any person who is not afflicted in soul on that same day shall be cut off from his people. “And any person who does any work on that same day, that person I will destroy from among his people. “You shall do no manner of work; it shall be a statute forever throughout your generations in all your dwellings. “It shall be to you a Sabbath of solemn rest, and you shall afflict your souls; on the ninth day of the month at evening, from evening to evening, you shall celebrate your sabbath.”

Looks like God himself changed the day! By giving them another Sabbath rest, but they kept it just like the 7th day Sabbath. The same penalty was enforced for violating the day just as the Sabbath. This is because the Sabbath is not just limited to the 7th day, it's meaning is to rest and there were other Holy Days called the Sabbath.

Lev. 23:36 `For seven days you shall offer an offering made by fire to the LORD. On the eighth day you shall have a holy convocation, and you shall offer an offering made by fire to the LORD. It is a sacred assembly, and you shall do no customary work on it. Just like a Sabbath another day of worship but this is on Sunday. All the feast days were Holy Convocations.

The first day there shall be a sabbathrest, and on the eighth day a sabbathrest. On the first day there shall be a sabbathrest, and on the eighth day a sabbathrest. Lev 23:41 `You shall keep it as a feast to the LORD for seven days in the year. It shall be a statute forever in your generations. You shall celebrate it in the seventh month. 7th day Adventists break this, yet this is a command just like the Sabbath is. The fact is the Sabbath is a day of rest, the other feast days were assemblies of worship. They have it backwards!

Questions for Sabbatarians on worship

Matt. 2:11: “And when they had come into the house, they saw the young Child with Mary His mother, and fell down and worshiped Him. And when they had opened their treasures, they presented gifts to Him: gold, frankincense, and myrrh.” Was this the Sabbath?

There were numerous times Jesus received worship from his disciples not once does it mention what day it was. We can assume many of these days were not the Sabbath since they were not congregated in the temple.

Matt. 8:2: “And behold, a leper came and worshiped Him, saying, “Lord, if You are willing, You can make me clean.” Was this the Sabbath?

Matt. 9:18: “While He spoke these things to them, behold, a ruler came and worshiped Him, saying, “My daughter has just died, but come and lay Your hand on her and she will live.” Was this the Sabbath?

Matt. 14:33: “Then those who were in the boat came and worshiped Him”

Can you prove all of these were on Saturday only, If not then they were breaking the commandment.

Matt. 28:1: “Now after the Sabbath, as the first day of the week began to dawn.”

Now after - in the end (referring to the Sabbath) Greek word is compose from the same as 3694 (through the idea of backwardness); (adverbially) late in the day; by extension, after the close of the day:

First in Greek: mia means one or first:, it is applied to the first day AFTER the SABBATH, the first day of the week (which is Sunday).

Matt. 28:1:  “Now after the Sabbath, as the first day of the week began to dawn, Mary Magdalene and the other Mary came to see the tomb…v:9 And as they went to tell His disciples, behold, Jesus met them, saying, “Rejoice!” So they came and held Him by the feet and worshipped Him.

This was on a Sunday the first day of the week - the resurrection day. If worshipping was not allowed on Sunday, Jesus would have rebuked them! But he didn't. Considering that this is right after the resurrection would be a right time to set them straight on worship days, but nothing is said.

Matt. 28:17: “When they saw Him, they worshiped Him; but some doubted.” A new way to worship is established, as Jn.4:24 says to “worship in spirit and in truth,” not just on a certain day. It no longer was a certain day or place but in truthfulness from the heart not from the rituals of the law. We can now come boldly before his throne of grace.

Luke 24:13-:  “Now behold, two of them were traveling that same day to a village called Emmaus, which was about seven miles from Jerusalem.   And the one of them, whose name was Cleopas, answering said unto him, Art thou only a stranger in Jerusalem, and hast not known the things which are come to pass there in these days? V:21 But we trusted that it had been he which should have redeemed Israel: and beside all this, to day is the third day since these things were done. This means it was the Sunday the day of resurrection. ...: But they constrained him, saying, Abide with us: for it is toward evening, and the day is far spent. And he went in to tarry with them.   And their eyes were opened, and they knew him; and he vanished out of their sight.  And they said one to another, Did not our heart burn within us, while he talked with us by the way, and while he opened to us the scriptures?  And they rose up the same hour, and returned to Jerusalem, and found the eleven gathered together, and them that were with them, …V.36 And as they thus spake, Jesus himself stood in the midst of them, and saith unto them, Peace be unto you” V:52 And they worshiped Him, and returned to Jerusalem with great joy, on a day which certainly seems to be Sunday or during the week, not on the Sabbath! V:53 And were continually in the temple, praising and blessing God. Amen.” Not just on the Sabbath but continually. Are we to believe that when Sunday came they left the temple and no one was inside?

Jesus received worship from Thomas (John 20:27-28). If we go back to v. 19 we find "Then, the same day at evening, being the first day of the week," Jesus did not stop the disciples from worshiping him, he accepted it. We then read in John 20:26 "And after eight days His disciples were again inside, and Thomas with them."
Thomas sees the risen Lord and one of the greatest statements of worship is spoken, v.28 "My Lord and my God!"

What Sabbatarians want us to believe is that we can worship any day, but Saturday is the official day and Sunday is forbidden! Where does it ever say this? Are we to believe we can congregate every day except Sunday! When sins are listed in the New Testament not once is Sabbath keeping listed. (Example: Galatians 5:19-21 has a list of 15 sins.) The neglect of Sabbath keeping is never condemned as sin in the New Testament.

Acts 2:1: “Now when the Day of Pentecost had fully come, they were all with one accord in one place.” This in relation to psalm 133 the brethren dwells together in unity, this is an assembly of worship and reading the scriptures. This was on a Sunday! This is the day when he first appeared after the resurrection so they continued in remembrance of him. Are we to believe that they were were disobeying him by worshipping on the wrong day and God filled with the Holy Spirit! Jesus' resurrection appearance prompted the Church to change the day of gathering from the Sabbath to Sunday for numerous reasons. Most importantly,  Israel as a nation stayed in unbelief, they as individual Jews now believed. The separation of Judaism from its old dispensation was a necessity since the gospel was now centered on the death and the resurrection of Christ. Israel continued to keep all the days the sacrifices and the kosher foods as obligation to the law. The Church learned her new found freedom from the Old Testament law covenant to keep anything from the Old Testament covenant as a choice.

It was Joshua who said after the law was given, “choose you this day who you will serve, as for me and my house we will serve the Lord.”

According to the Sabbatarian interpretations this would read more like on this day only we serve, we will serve on the Sabbath. Joshua's point was for Israel to make a decision that day if they will serve God, not which day they will worship.

When the Samaritan woman said to Jesus: Our fathers worship on this mountain and you Jews say that in Jerusalem is the place where one ought to worship. Jesus said to her “but the hour is coming, and now is, when the true worshippers will worship the father in Spirit and truth.” meaning that our heart will be free to worship at any time in the correct way. It's not the place we worship, nor the day, otherwise we would need to go to the temple still (not a church).

Its not the WHEN we worship, but the WHO we worship!

 If one wants to congregate to worship on Saturday or Sunday, or even Monday they have the freedom under the new covenant to do so. Please don't insult our intelligence or distort the Bible's instructions to perpetuate the myth that Sunday is the Mark of the Beast, a replacement of Saturday. It is not. Worship is a way of life, not a particular day of assembly over another day.

New Testament believers should not allow themselves to be taken captive by legalists of the law.