The easiest way to find out the time when a particular post office closes is to visit the main USPS website. It has a link for finding individual locations that displays the hours for each branch in the United States.
The hours of operation for each USPS branch vary depending on the specific location and the day of the week. There is no universal time when all United States post office locations close for business each day. Plus, some branch locations are open on Saturdays while others aren't. Since there is no mail service on Sundays, all USPS locations are closed for business on this day.