FRCP Rule 6
Federal Rule of Civil Procedure 6 governs computing time periods in federal court.
FRCP 6(a) governs how to count time when a rule, statute, or court order specifies a time period. For periods stated in days: exclude the triggering day, count every calendar day including weekends and holidays, and if the last day is a weekend or legal holiday, the period continues to the next non-weekend, non-holiday.
FRCP 6(d) adds 3 days when service is made by mail.
See the worked examples in the FRCP 6 court-day calculation guide.