From what I’ve studied, Constantine did NOT “fobid” observance of the sabbath. He merely said that workshops and other activities be closed on Sunday.
From the very beginning, Christians NEVER worshipped the Lord on Saturday. That should not be in dispute. They came together on the Lord’s Day (first day of the week) but also kept the Jewish Sabath. One can cast dispersions on ANY DAY by associating it with what pagans happen to do on that day but it is probably not coincidental that the Lord specifically chose the first day of the week for the ressurection and to baptize the church with the Holy Spirit.
All that said. I think believers who place greater emphasis on their particular day of worship moreso than the creator of it are on very dangerous turf. Has it become a “law unto itself” and does it resemble the attitude for which Jesus chastized the scribes and pharisees? EVERY DAY is a day when believers should study the word, pray, meditate on the things of God and, most importantly, live in accordance with it. Neither are we limited on the assembling of ourselves together on any particular day.
Whether Saturday or Sunday is recognized as the sabbath is, in my opinion, of little consequence. The day a believer chooses to assemble themselves together will NOT be the determining factor for eternity. What matters for Christians, is Christ’s attonement and the salvation available through Him. Anything else is merely religion that will be separated with the chaff. All those who are saved by Him will be together with Him and the Bible says the saints will praise God continually (not just at some time reserved for it).