5 C.F.R. § 1201.23
In computing the number of days allowed for complying with any deadline, the first day counted is the day after the event from which the time period begins to run. If the date that ordinarily would be the last day for filing falls on a Saturday, Sunday, or Federal holiday, the filing period will include the first workday after that date. Unless a different deadline is specified by the Board or its designee, 5 days are added to a party's deadline for responding to a document served on the party by mail.
Example 1: If an employee receives a decision notice that is effective on July 1, the 30-day period for filing an appeal starts to run on July 2. The filing ordinarily would be timely only if it is made by July 31. If July 31 is a Saturday, however, the last day for filing would be Monday, August 2.
Example 2: The judge orders the appellant to file a response to a jurisdictional order no later than October 15, 2012, and that the agency's response is due 10 days after the filing of the appellant's pleading. If the appellant serves the agency with a pleading via regular mail on October 15, the agency's deadline for filing a response will be October 30, not October 25.
5 C.F.R. §1201.23