35 U.S.C. 21(a)

The Director may by rule prescribe that any paper or fee required to be filed in the Patent and Trademark Office will be considered filed in the Office on the date on which it was deposited with the United States Postal Service or would have been deposited with the United States Postal Service but for postal service interruptions or emergencies designated by the Director.

37 C.F.R. §2.195(d) Interruptions in USPS

If the Director designates a postal service interruption or emergency within the meaning of 35 U.S.C. 21(a), any person attempting to file correspondence by Priority Mail Express® Post Office to Addressee service who was unable to deposit the correspondence with the USPS due to the interruption or emergency may petition the Director to consider such correspondence as filed on a particular date in the Office. The petition must:

1. (1) Be filed promptly after the ending of the designated interruption or emergency;
2. (2) Include the original correspondence or a copy of the original correspondence; and
3. (3) Include a statement that the correspondence would have been deposited with the USPS on the requested filing date but for the designated interruption or emergency in Priority Mail Express® service; and that the correspondence attached to the petition is the original correspondence or a true copy of the correspondence originally attempted to be deposited as Priority Mail Express® on the requested filing date.

Under 35 U.S.C. §21(a) and 37 C.F.R. §2.195(d), if the Director designates a postal interruption or emergency, any person who attempted to file correspondence by Priority Mail Express® Post Office to Addressee service but was unable to deposit such correspondence with the USPS due to the postal interruption or emergency may petition the Director to consider the correspondence to have been filed in the USPTO on a particular date.

The petition must:

1. (1) be filed promptly after the ending of the designated interruption or emergency;
2. (2) include the original correspondence or a copy of it; and
3. (3) include a statement that such correspondence would have been filed on that particular date if it were not for the interruption or emergency in Priority Mail Express® service, and that the correspondence attached to the petition is the original correspondence or a true copy of it originally attempted to be deposited as Priority Mail Express® on the requested filing date.

37 C.F.R. §2.195(d)(1)-(3).