Some USPS Priority Mail and First Class Mail packages arrive within 1-3 business days, while others take longer to be delivered.
If the tracking number shows that the package is "In Transit, Arriving Late" that means that the package will be delivered later than expected.
USPS closed 150 processing plants in 2014 and another 82 more in 2015, and the remaining processing plants are having a difficult time keeping up with the volume of mail that they receive. On top of these processing plant closings, the new Postmaster General has prohibited postal workers from working overtime and removed automated sorting equipment from USPS facilities. These plant closing, overtime prohibitions, and equipment removals have significantly lowered the percentage of mail that is delivered on time.