The US postal service does seem to be delivering packages on Sunday (as of now). I've recently started having the USPS "Parcel Select Lightweight" packages shipped from Amazon.com that were slated for Monday, getting delivered on Sunday at around 1pm.
In the past, it seemed like the US postal service never delivered on Sunday. There is no delivery Holidays. However, mail processing and transporation goes on everyday of the year ( 24/7 365 days a year).