While the church "also" gathered together on the first day of the week (Sunday), they attended Synagogue on the 7th. (Saturday/Sabbath). There is worship of God during a synagogue service. It may look and sound different from what many call worship of God, but it is worship.
It depends on how you define worship. Most people today would probably consider synagogue services to be worship, I would probably agree that today they are.
However, synagogues did not come into existence until the Babylonian captivity and were not an original fixture of Judaism as it was delivered from God. They came into existence because the Jews were deprived of the temple and the synagogue became a way to carry on with what aspects of the religion that could be exercised without the temple.
Strictly speaking under Jewish law no 'worship' was to be done outside the temple. This is holding the view that worship is basically synonymous with sacrifice. Thus, technically when the temple was in existence synagogues were not places of worship, but rather places of teaching and prayer (which I believe I said many Christians continued to do on the sabbath, in the form of bible study and reflection. If I didn't say it I intended to, along with the point that the Christians did not consider this to be a legalistic keeping of the sabbath, or something that people were required to do.)
I'm assuming that you are confused about this link. It is not a link to Catholic material but in fact a link to 7th day adventist argument. The quotes listed on the site are not original source material quotes but rather quotes from text books and the like. The point of the site is to try and prove that the 7th day view point didn't originate in the 1800's but has earlier roots.
The fact is sunday worship was not instituted by any Pope, or by Constantine. It existed as the primary celebration of the Church from the very beginning.
Acts 15:21 says, "For Moses of old time hath in every city them that preach him, being read in the synagogues every sabbath day".
In context with the Acts notation above, Peter's suggestion was to write out for the new believers 4 things they were to abstain from, with the conclusion they would learn about Moses when they heard him in the synagogue every Sabbath day.
Any city with a large enough Jewish community would have a synagogue and the people who lived within the city would go to the synagogue on the sabbath. The synagogue, first of all, did not conduct sacrificial worship which was at the heart of the Jewish faith, and is at the heart of the Christian faith.
Christians did continue to go to synagogues on sabbath until they were forcibly thrown out by the Jews. Even after that many Christians (those who were able) devoted the sabbath to study and prayer. It was not, however, a requirement.
Further, any Jewish family which lived more than 2 miles from a synagogue would have been forbidden by the law from going to synagogue on the sabbath. 2 miles is actually the extended distance that came into use because of the pharisees extending the previous distance. The first specific distance given was 1 mile total, then it was extended to 2 miles total, then up to 4 miles total round trip.
The actual biblical command given by God was that no one was allowed to "go out of their place" on the sabbath. Much later the Rabbinical tradition interpreted 'his place' to mean the city in which he lived.
How early? I don't see this to be the consensus among the believers in the Bible.
Justin Martyr wrote about it around 150 AD. I've seen it talked about in other sources as well but he is the only one I can remember specifically off hand.
For reference, here is what the Early Christians taught about the sabbath, from Ignatius of Antioch. This guy lived during the life time of the apostles. He was a student of the apostle John, and was ordained bishop of Antioch by the apostles.
If, therefore, those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord's Day, on which also our life has sprung up again by Him and by His death - whom some deny, by which mystery we have obtained faith....Let us not, therefore, be insensible to His kindness...Therefore, having become His disciples, let us learn to live according to the principles of Christianity. For whosoever is called by any other name besides this, is not of God. Lay aside, therefore, the evil, the old, the sour leaven....It is absurd to profess Christ Jesus, and to Judaize. For Christianity did not embrace Judaism, but Judaism Christianity... Ignatius - To the Magnesians 9-10
This was written sometimes between 110 -120 AD, 50 to 60 years after Paul and Peter died, roughly and maybe about 10 years after John died (if he did
) It was written by Ignatius shortly before his own death by Martyrdom. In fact he was on his way to be executed in Rome when he wrote this.
*edit* I was incorrect in the dates on Ignatius. His Martyrdom took place between 98 AD and 117 AD, most likely in 108 AD, thus the letter above was probably written in 108 AD. By comparison, Peter and Paul were Martyred around 66-68 AD. John was released from his exile on Patmos around 96 AD (there is no clear recorded death date for John).
Also, I don't think Ignatius was condemning study or worship on the sabbath, or going to a synagogue necessarily. He was speaking about those who believe it is a requirement to keep the sabbath in the context of the Mosaic covenant of Sinai.
*2nd Edit*
The early Christians, such as Ignatius, and Justin Martyr did NOT consider sunday to be a new sabbath. They didn't advocate keeping the sabbath at all in a legal sense. They didn't believe that the sabbath changed from saturday to sunday.
I think that in much of the Church that general idea (sunday is the new sabbath) did eventually become prevalent and the development of that idea may be traced to things like Constantine's sunday law etc. However, this had not so much to do with worshipping on sunday as it did with the sense of legalism which alway seems to accompany every group to some degree.
In more detail what Constantine did in his sunday decree was declare that all the courts (government offices) would be closed on sunday and decreed that slaves shouldn't be forced to work on sunday, with some exceptions for farmers.
Here is the actual decree from Constantine. It wasn't a formal law per say.
On the venerable day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed. In the country however persons engaged in agriculture may freely and lawfully continue their pursuits because it often happens that another day is not suitable for grain-sowing or vine planting; lest by neglecting the proper moment for such operations the bounty of heaven should be lost