Informal is just that--informal. Different sororities hand out bids differently. Chances are, you'll get a call after Saturday or you may even get a bid on Saturday--it's all up to the sorority.
It gets a little hairy when we have girls pursuing infomal recruitment in different rooms when another group decides to hand out bids earlier than us, but that's precisely that--the sororities set their own schedules.
Good luck from one Bear to a different Bear!