Computation of any period of time referred to in sections 1-21j-1 to 1-21j-57, inclusive, of the Regulations of Connecticut State Agencies begins by first counting the day after the day on which the precipitating event occurs, and ends on the last day of the period so computed. The last day of the period is to be included unless it is a day on which the principal office of the commission is closed, in which event the period shall run until the end of the next following business day. If the period of time, including the intervening Saturdays, Sundays and legal holidays, is five (5) days or less, such Saturdays, Sundays and legal holidays shall be excluded from the computation; otherwise such days shall be included in the computation.