In computing any period of time prescribed or allowed by this part, by notice or order of a law judge or the Board, or by any applicable statute, the date of the act, event or default after which the designated period of time begins to run is not to be included in the computation. The last day of the period so computed is to be included unless it is a Saturday, Sunday or legal holiday for the Board, in which event the period runs until the end of the next day which is not a Saturday, Sunday or legal holiday. In all cases, Saturdays, Sundays and legal holidays for the Board shall be included in the computation of time, except they shall not be included in computations of time respecting petitions for review of determinations as to the existence of emergencies under §821.54.