How are Priority Mail Express® submissions dated by the USPTO?

Priority Mail Express® submissions to the USPTO are treated differently in terms of dating. According to 37 CFR 1.6(a)(2):

Correspondence filed in accordance with § 1.10 will be stamped with the date of deposit as Priority Mail Express® with the United States Postal Service.

This means that new patent applications filed using Priority Mail Express® will be stamped with the date they were deposited with USPS, even if that date is a Saturday, Sunday, or federal holiday. The MPEP provides an example:

For example, if a new patent application is deposited in Priority Mail Express® in accordance with 37 CFR 1.10 on a Saturday and the United States Postal Service gives it a date of deposit of Saturday, the Office will accord and stamp the correspondence with the Saturday date.