37 C.F.R. 1.6 Receipt of correspondence.

1. (a) Date of receipt and Express Mail date of deposit. Correspondence received in the Patent and Trademark Office is stamped with the date of receipt except as follows:
   2. (1) The Patent and Trademark Office is not open for the filing of correspondence on any day that is a Saturday, Sunday, or Federal holiday within the District of Columbia. Except for correspondence transmitted by facsimile under paragraph (a)(3) of this section, or filed electronically under paragraph (a)(4) of this section, no correspondence is received in the Office on Saturdays, Sundays, or Federal holidays within the District of Columbia.
   3. (2) Correspondence filed in accordance with § 1.10 will be stamped with the date of deposit as "Express Mail" with the United States Postal Service.
   4. (3) Correspondence transmitted by facsimile to the Patent and Trademark Office will be stamped with the date on which the complete transmission is received in the Patent and Trademark Office unless that date is a Saturday, Sunday, or Federal holiday within the District of Columbia, in which case the date stamped will be the next succeeding day which is not a Saturday, Sunday, or Federal holiday within the District of Columbia.
   5. (4) Correspondence may be submitted using the Office electronic filing system only in accordance with the Office electronic filing system requirements. Correspondence submitted to the Office by way of the Office electronic filing system will be accorded a receipt date, which is the date the correspondence is received at the correspondence address for the Office set forth in § 1.1 when it was officially submitted.

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The United States Patent and Trademark Office (Office) stamps papers and fees received through the mail (other than those submitted by Express Mail in accordance with 37 CFR 1.10), by facsimile, or via EFS-Web with the date of their receipt in the Office. Papers that are hand-carried to the Office will be date stamped at the Customer Service Window. See MPEP § 502. The stamp is referred to as the "Office Date" stamp.

When the last day for taking any action or paying any fee in the Office falls on a Saturday, Sunday, or a Federal holiday within the District of Columbia, the action or the fee is considered timely if the action is taken or the fee is paid on the next succeeding business day.

New patent applications filed in accordance with 37 CFR 1.10 will be stamped by the Office with the date of deposit as "Express Mail" with the United States Postal Service. For example, if a new patent application is deposited in "Express Mail" 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. 37 CFR 1.6(a)(2).

Similarly, correspondence submitted via the USPTO electronic filing system (EFS-Web) will be accorded a receipt date on the date the correspondence is received at the correspondence address for the Office set forth in 37 CFR 1.1 (i.e., local time in Alexandria, VA), without regard to whether that date is a Saturday, Sunday, or Federal holiday within the District of Columbia. See MPEP §§ 502.05 and 512.

If an application as originally filed includes the necessary components for a filing date (see 37 CFR 1.53(b) - (d) and MPEP § 506), the "Office Date" stamp establishes the "filing date."