Acts 17:1

"When they had passed through Amphipolis and Apollonia, they came to Thessalonica, where there was a Jewish synagogue." NIV translation

Paul & Silas had been released from prison in Philippi, (see chapter 16), and were requested to leave the city. We are told here that they passed through Amphipolis and Apollonia on the way to Thessalonica. Amphipolis, whose name means an all round city, was an unusual city as it was round instead of square. 

(Most cities were built with square walls around them.) The city was about three miles from the Aegean Sea, Apollonia was to its West and Thessalonica was about thirty eight miles West of Apollonia. Thessalonica had three rivers near it that flowed into the sea and it was a larger city as we know due to the fact that it had a synagogue. (The formation of a synagogue required there to be at least ten adult Jewish men.)

Acts 17:2 & 3

"As his custom was, Paul went into the synagogue, and on three Sabbath days he reasoned with them from the Scriptures, explaining and proving that the Christ had to suffer and rise from the dead. 'This Jesus I am proclaiming to you is the Christ,' he said." NIV translation

Paul did his usual thing in that he went into the local synagogue and shared the Word of God. Usually, some would believe in Jesus while most of them rejected the teaching and forced him out to the Gentiles. That is how most of the local bodies of believers ("churches") were started. He spent less than a month speaking in the synagogue because we are told that he preached on only three Sabbath days. (Paul was not called to put roots down for himself in any one place. He was called to travel and preach the Word.) His message was always consistent in that it was about the death of Christ and the fact that He rose from the dead.