In computing any period of time prescribed or allowed by this subchapter, the date of the act, event, or default after which the designated period of time begins to run is not to be included. The last day of the period so computed is to be included, unless it is a Saturday, a Sunday, or a legal holiday, in which event the period runs until the end of the next day which is not a Saturday, a Sunday, or a legal holiday.
28 Tex. Admin. Code § 1.803