Yes USPS mail is picked up on Saturday around 4:00 PM to 4:30 PM at your local USPS drop box or USPS post office location.
Your mail should be picked up during the normal delivery hours if your mail is delivered or picked up at your address at a home USPS box or delivered to your door.
Saturdays are working days for USPS so if mail gets delivered it will get picked up on Saturday as well.
USPS mail is picked up here at 4:30 PM on Saturday like it is on Monday through Friday.