Albert Barnes on Acts 16:13

And on the sabbath. There is no doubt that in this city there were Jews. In the time of the apostles they were scattered extensively throughout the known world.

By a river side. What river this was is not known. It is known, however, that the Jews were accustomed to provide water, or to build their synagogues and oratories near water, for the convenience of the numerous washings before and during their religious services.

Where prayer. Where there was a proseuchae, or place of prayer; or where prayer was commonly offered. The Greek will bear either; but the sense is the same. Places for prayer were erected by the Jews in the vicinity of cities and towns, and particularly where there were not Jewish families enough, or where they were forbidden by the magistrate to erect a synagogue. These proseuchae, or places of prayer, were simple enclosures made of stones in a grove, or under a tree, where there would be a retired and convenient place for worship.

Was wont. Was accustomed to be offered; or where it was established by custom.

And spake unto the women, etc. This was probably before the regular service of the place commenced.

(2) "the sabbath" "Sabbath day" (e) "was wont" Acts 21:5