Sunday worship comes from the early church wanting to separate themselves from "judaizers" by complementing the worship practices of Constantine, who was a sun worshiper who legalized Christianity in 313 and referred to Sunday as “the venerable day of the sun” in 321 AD when it was first instituted.
The Laodicean Council (363-364 AD) Canon 29: "Christians must not judaize by resting on the [Jewish] Sabbath, but must work on that day, rather honoring the Lord’s Day; and, if they can, resting then as Christians. But if any shall be found to be judaizers, let them be anathema (excommunicated) from Christ."
This means that they are not only willing to knowingly forsake the known Sabbath, but are saying Christians
must work at that time.
Note that they still consider Saturday to be the Sabbath, though they don't keep it.
Canon 16: The Gospels are to be read on the
Sabbath [i.e. Saturday], with the other Scriptures.
Cardinal James Gibbons, The Faith of Our Fathers (Ayers Publishing, 1978 ): 108: "But you may read the Bible from Genesis to Revelation, and you will not find a single line authorizing the sanctification of Sunday. The Scriptures enforce the religious observance of Saturday, a day which we never sanctify."
This is why Catholics like James Gibbons laugh at Protestants who still hold Sunday as a 'holy day' and also discount the Sabbath instituted in canonical Scripture (Saturday). They see the wide acceptance for Sunday as the Sabbath as evidence of their authority over Scripture being affirmed by the very denominations that disagrees with them.
Also of note, the keeping the Sabbath is
not just a Mosaic Law, as it can first be seen immediately after creation in
Genesis 2:1-3. This means it wasn't just for the jews (who didn't exist yet), but for creation as a whole.
Sun worship continues to be a fixture in the Catholic church to this day (the use of the monstrance, the solar wheel in the Vatican, etc.). This has been similarly done most notably with Christmas.
I ONLY BRING THIS UP AS IT IS RELEVANT TO THE CONNECTION BETWEEN SUN WORSHIP AND THE EARLY CHURCH.
Dies Natalis Solis Invicti in the old Julian calendar was Sol Invictus’ birthday (sun god) and was celebrated on December 25th. The Temple of the Sun was made in 274 by Aurelian on December 25th, following a successful campaign against Palmyra in 272 (funded by the spoils of that campaign).
During Constantine’s reign the cult of Sol Invictus had a feast day on December 25th. The first official record of Jesus’ birthday being celebrated on December 25th was in 336, the year before Constantine died.
Special Mention: Sextus Julius Africanus has Jesus’
conception date as December 25th since, at the time, Jesus was thought to have died on March 25th and it is a
jewish belief that prophets die on the day they were born (or according to Sextus’ interpretation, conceived).