USPS packages and letters and all mail through the USPS is moving 24 hours per day 7 days per week even on the weekends or days when the regular USPS post office is closed.
The mail through USPS is always being sorted at sorted facilities around the clock even on holidays so that your mail will be delivered on time during normal business days and normal business hours at your post office.
The USPS drivers hauling the mail in tractor trailers as well as airplanes are always working on weekends and holidays to get the mail to where it needs to be.
So while your local USPS is closed the sorting facilities are still open day and night all week long to sort all that mail that is being sent and received.