Tex. Election Code § 13.143

(a) Except as provided by Subsections (b) and (e), if an applicant's registration application is approved, the registration becomes effective on the 30th day after the date the application is submitted to the registrar or on the date the applicant becomes 18 years of age, whichever is later.

(b) A registration is effective for purposes of early voting if it will be effective on election day.

(c) A registration is effective until canceled under this code.

(d) For purposes of determining the effective date of a registration, an application submitted by:

(1) mail is considered to be submitted to the registrar on the date it is placed with postage prepaid and properly addressed in the United States mail; or

(2) telephonic facsimile machine is considered to be submitted to the registrar on the date the transmission is received by the registrar, subject to Subsection (d-2).

(d-1) The date indicated by the post office cancellation mark is considered to be the date the application was placed in the mail unless proven otherwise.

(d-2) For a registration application submitted by telephonic facsimile machine to be effective, a copy of the original registration application containing the voter's original signature must be submitted by personal delivery or mail and be received by the registrar not later than the fourth business day after the transmission by telephonic facsimile machine is received.

(e) If the 30th day before the date of an election is a Saturday, Sunday, or legal state or national holiday, an application is considered to be timely if it is submitted to the registrar on or before the next regular business day.