Acts 18:4
And he reasoned in the synagogue every sabbath, and persuaded the Jews and the Greeks.
These where mission trips or for the purpose of witnessing. It is by no means the same as early Christians setting aside a day to gather and worship collectively. The synagogues were not churches it was hostile ground that Christ warned of but seeing we are sheep led to the slaughter they were rich mission fields as many peoples were gathered there.
Acts 17:1 Now when they had passed through Amphipolis and Apollonia, they came to Thessalonica, where was a synagogue of the Jews:
Acts 17:10 And the brethren immediately sent away Paul and Silas by night unto Berea: who coming
thither went into the synagogue of the Jews.
The synagogues were not churches and were not church gatherings they were Jewish places of meeting and the apostles witnessing to Jews and Greeks alike.
Matthew 10:5-8 These twelve Jesus sent forth, and commanded them, saying, Go not into the way of the Gentiles, and into
any city of the Samaritans enter ye not:
6But go rather to the lost sheep of the house of Israel.
7And as ye go, preach, saying, The kingdom of heaven is at hand.
8Heal the sick, cleanse the lepers, raise the dead, cast out devils: freely ye have received, freely give.
Matthew 10:16-17 Behold, I send you forth as sheep in the midst of wolves: be ye therefore wise as serpents, and harmless as doves.
17But beware of men: for they will deliver you up to the councils, and they will scourge you in their synagogues;
Luke 21:12-13 But before all these, they shall lay their hands on you, and persecute
you, delivering
you up to the synagogues, and into prisons, being brought before kings and rulers for my name's sake.
13And it shall turn to you for a testimony.
They kept the sabbath for the purpose to witness. However, their gathering together as the church was on the first day of the week for them to collectively worship God. I personally believe that whatever day a Christian considers Holy between him and God let him do as his conscience dictates.