Why did the group meet by the river in Philippi?
The group met by the riverside in Philippi because the city apparently lacked a full synagogue. The Roman authorities had likely permitted them to gather at a park along the river for prayer and devotions, and on the Sabbath Paul and his companions joined the women who resorted there.
Les explains that Philippi did not have a full-blown synagogue, so the Roman authorities probably gave permission for a small group to meet out at a park along the river, where they could have their devotions. On the Sabbath, Paul and Silas went out of the city by a riverside, where prayer was wont to be made, and sat down and spoke to the women who resorted thither, including Lydia, a seller of purple from Thyatira.