I'm sure the dates were announced well in advance, but the advertising just recently went up that the Koshigaya Awa-Odori festival will be held August 22, 23, and 24. They had a preview of sorts on the 27th.
August 22nd, events take place 19:00-21:00. August 23rd and 24th, there are events starting around 14:00. Details for the event and a few photos from prior years are on the website (Japanese). There are plenty of stalls set up in the streets selling food and there are some carnival games.
I'm planning on attending in the evening; I have to work all three days but I should be able to see a little of the festival each day.