Worshiping on 1st day/8th day is all over the place.
Leviticus 23
35‘On the first day is a holy convocation; you shall do no laborious work of any kind.
36‘For seven days you shall present an offering by fire to the LORD. On the eighth day you shall have a holy convocation and present an offering by fire to the LORD; it is an assembly. You shall do no laborious work.
Leviticus 23
39‘On exactly the fifteenth day of the seventh month, when you have gathered in the crops of the land, you shall celebrate the feast of the LORD for seven days, with a rest on the first day and a rest on the eighth day.
Nehemiah 8:18
18He read from the book of the law of God daily, from the first day to the last day. And they celebrated the feast seven days, and on the eighth day
there was a solemn assembly according to the ordinance.
Ezekiel 43
27‘When they have completed the days, it shall be that on the eighth day and onward, the priests shall offer your burnt offerings on the altar, and your peace offerings; and I will accept you,’ declares the Lord GOD.”
Leviticus 14
10“Now on the eighth day he is to take two male lambs without defect, and a yearling ewe lamb without defect, and three-tenths
of an ephah of fine flour mixed with oil for a grain offering, and one log of oil;
11and the priest who pronounces him clean shall present the man to be cleansed and the aforesaid before the LORD at the doorway of the tent of meeting.
Leviticus 14
23“Then the eighth day he shall bring them for his cleansing to the priest, at the doorway of the tent of meeting, before the LORD.
24“The priest shall take the lamb of the guilt offering and the log of oil, and the priest shall offer them for a wave offering before the LORD.
Acts 15
21“For Moses from ancient generations has in every city those who preach him, since he is read in the synagogues every Sabbath.”
James had to explain to gentiles what Jews did on the Sabbath. Otherwise they would have been doing the same thing, like reading scriptures.