Jamieson, Fausset and Brown on Acts 16:16-18

16-18. as we went to prayer--The words imply that it was on their way to the usual place of public prayer, by the river-side, that this took place; therefore not on the same day with what had just occurred.

a ... damsel--a female servant, and in this case a slave (Ac 16:19).

possessed of a spirit of divination--or, of Python, that is, a spirit supposed to be inspired by the Pythian Apollo, or of the same nature. The reality of this demoniacal possession is as undeniable as that of any in the Gospel history.