When mail goes to a USPS distribution center the mail is sorted the same day and then leaves the sorting facility or distribution center that night or early morning.
When you mail something at your post office it goes to a major distribution center where the mail is then sorted and then the mail either goes out of state to another USPS distribution center closest to the address.
Or if you're in the same state the mail may leave the USPS distribution center that night or morning and then head to your local post office or a post office closest to your address where it will then be delivered.
Mail does not stay in a USPS distribution center longer than one day.
When mail goes to a USPS distribution center it leaves the same night or next morning.
The mail will not stay in the USPS distribution center longer than a day because the mail is then sorted and put onto a truck for shipping.
Thousands of packages and mail are being sorted and shipped daily and the USPS distribution centers stay open on weekends and holidays as well.