How does the USPTO handle correspondence received on weekends or holidays?

The USPTO does not receive or process correspondence on Saturdays, Sundays, or Federal holidays within the District of Columbia, with some exceptions. According to 37 CFR 1.6(a)(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.

When the last day for taking action or paying a fee falls on a weekend or holiday, it’s considered timely if done on the next business day: 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.