Acts 16:13

And on the sabbath day we went forth without the gate by a river side, where we supposed there was a place of prayer; and we sat down, and spake unto the women that were come together.

On the sabbath day ... There was no synagogue in Philippi, but whatever Jews might have been in the area could have been expected to observe prayers on the sabbath, and the preachers "supposed" such a place of prayer to be a certain site on the river's edge. If Luke had been a citizen of Philippi, as many have believed, it is rather strange that he would not have known certainly of this place of prayer. Bruce commented that "From this time Luke apparently spent some years in Philippi."[9] The fact of there having been no synagogue means that there were fewer than ten Jewish men living in Philippi, that being the number required before a synagogue could be built.[10] In the absence of a synagogue, the Jews often provided places of prayer by the rivers, or other suitable locations, the custom of going to the rivers for these sites dating from the Babylonian captivity (Psalms 137:1; Ezra 8:15,21). "Claudius had banished the Jews from Rome and therefore from the colonies (Acts 18:2), and it may be that Philippi had obeyed this order." The river in this case was the Gangites.

[9] F. F. Bruce, The Book of Acts (Grand Rapids, Michigan: Wm. B. Eerdmans, Publishers, 1954), p. 330.

[10] H. Leo Boles, op. cit., p. 256.