They were there for the same reason Paul eventually went to synagogues after his conversion....to convert Jews to Christianity:
Act 18:4 And he reasoned in the synagogue every sabbath, and persuaded the Jews and the Greeks.
Act 18:5 And when Silas and Timotheus were come from Macedonia, Paul was pressed in the spirit, and testified to the Jews that Jesus was Christ.
Act 20:7 And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.
Act 20:8 And there were many lights in the upper chamber, where they were gathered together.
It was a Sunday they gathered together for fellowship, food and preaching.