I have done my research on this but I have not found many definitive answers. So some of what I write here will be my “best guessing.”
Paul and friends were in new territory. Europe was different from the areas they had traversed to get there: very few of the cities and towns of ancient Greece had Jewish meeting places. These Jewish meeting places were not yet known as synagogues because the temple in Jerusalem was still standing and functioning (until 70 AD). These meeting places were usually built near flowing water so that community members could ritually clean themselves before their time of Scripture reading and prayer.
So, “Then on the Sabbath we went outside the city gate to a place by the river, where we thought there would be a Jewish meeting place for prayer.”(Acts 16:13) Were Paul and friends looking for a building or just for a spot along the river where Jews might meet for prayer? They seem to have found neither. There were some women who had come. Had they come to pray or to wash clothes? In the home where I grew up Saturday was washing day. I do not know why the women were there but at least one of them received the Good News of Jesus Christ.
The video attached is of a baptismal spot created by the Greek Orthodox Church outside the city gate of ancient Philippi by the Zygaktis River. It was a beautiful spot – turn up your volume and listen to the water flow by.
Where do you go to pray? Perhaps we could us “washing day” as a time of prayer.