§ 20. Day, computation. A number of days specified as a period from a certain day within which or after or before which an act is authorized or required to be done means such number of calendar days exclusive of the calendar day from which the reckoning is made. If such period is a period of two days, Saturday, Sunday or a public holiday must be excluded from the reckoning if it is an intervening day between the day from which the reckoning is made and the last day of the period. In computing any specified period of time from a specified event, the day upon which the event happens is deemed the day from which the reckoning is made. The day from which any specified period of time is reckoned shall be excluded in making the reckoning.