The last mail pick up at the post offices is between 4:25 PM to 5:00 PM.
The actual time for last mail pick up at the post office depends on the post office location and what time they close and what time the mail truck comes around to pick up the mail.
Here the mail at the post office gets picked up by 4:25 PM and if you mail a letter or anything after 4:25 PM then the mail won't be picked up and go out until the next workday.
Mail is always traveling through the USPS sorting facilities and on the trucks through the day and night 24 hours per day and 7 days per week including Holidays so the mail gets to the destination when it's supposed too.
But the post offices only are open and sorting mail Monday through Saturday.
So if you want to make sure a piece of mail gets out the same day you mail it then you must mail it before 4:00 PM to ensure it gets out and picked up by the mail carrier.