When computing any period of time prescribed or allowed by these rules, the day of the act, event, or default from which the designated period of time begins to run shall not be included. The last day of the period so computed shall be included, unless it is a Saturday or Sunday, or a legal holiday, in which event the period runs until the end of the next day which is not one of the aforementioned days.
31 Tex. Admin. Code § 380.6