USPS does not usually deliver mail on Sunday.
However there are some exceptions to the Sunday Delivery of mail by USPS.
USPS does deliver certain Amazon packages and Priority Mail Express Packages on Sunday but most other mail is not delivered on Sunday.
USPS will usually only deliver mail on Sunday in locations such as a large city or large town.
If you're in a small town or get rural mail delivery then USPS will typically not deliver the mail on Sunday and you'll have to wait until Monday to get the mail.
The Postal Service currently delivers Priority Mail Express and certain Amazon packages on Sundays.
Also the USPS mail is still being sorted and being transported on Sunday and Holidays 24 hours per day and 7 days per week through the sorting facilities.
But most mail is not actually delivered to addresses or post office boxes on Sunday.
Normal USPS mail delivery is from Monday through Saturday.
And Saturday Mail delivery is usually not al day and stops sometimes around noon or 1:00 PM