Many studying the subject of Primitive Christianity think Sunday worship is valid ...however as a measure to separate themselves from lawless assemblies many have worship services on different days.
Now the problem with sects such as SDA is that they themselves alter the understanding of scriptures with repetitive affirmations, and boast that "the Sabbath is a day of WORSHIP" ...that is not so, scripture clearly states that the Sabbath is a DAY OF REST on which you shall do NO WORK (Note: Exodus 31:15)
So firstly they alter ones understanding by emphasising the falsity "Sabbath is a day of worship" when it clearly states "a day of rest" ...also in this causing many to stumble by having them perform services and many works to facilitate this worshipful sabbath service in causing many to work, and even "buy, sell, and trade."
Some find contradiction to this (in only resting, or assembled restings) ...but only because they confuse undefined statements with regular "Sabbath" days but are really referring to "Holy Sabbath" days making note of Holy convocations by ordnance of Yah ...however with the clear words showing a regular Sabbath day requirements written boldly in Torah ...in a regular Sabbath day it is clear what we should do.
Now those looking into Primitive Christian studies find themselves agreeing that the custom began from Christians by first keeping Sabbath the day before Sunday by resting, or resting together in large assembled groups of sleepovers, then on the following day performing all the great task of skillful worship ...which was on the following day, and first day, Sunday.
It was later on when the Assemblies were falling into lawlessness that they also forgot the Sabbath, as graphed in wild branches exulting themselves over the natural roots and tree, they began in their hateful pride started to only validate their own "solemn assemblies" in their minds as pleasing to YHVH ...when rebellion is as blasphemy in His eyes.
All in all, you get the point, if you are going to observe Sunday service ...also keep the Sabbath also, and even keep Sunday service with those who also keep Sabbath, that way you bring no stink upon your skillful praise of Him Who is worthy forever, and ever ...why labor in vain?