When all letters are sent.
The window, if open on a Saturday, may close at 11, 12, 4:00, or some other time.
Deliveries are made in the morning. Each route is assigned to a carrier or shared among many.
Any mail for the day, (when on the proper route), must be delivered that day. So unless you live in a small, remote PO area, deliveries end on the day all shipments are shipped.
Smaller areas usually don’t have problems in their offices causing overtime on Saturdays; from operator shortages, incoming calls, etc. Smaller Post Offices don’t have the delay state that larger offices have.
So the bigger the Post Office, in terms of routes, the more likely you are to receive your mail in the future.
