30 Del. C. § 551

(a) If any return, declaration of estimated tax, claim, statement, notice, protest or other document required to be filed, or any payment required to be made, within a prescribed period or on or before a prescribed date under the authority of any provision of the revenue laws of this State, other than § 331 of this title, is, after such period or such date, delivered by United States mail to the agency, officer or office with which such document is required to be filed, the date of the United States postmark stamped on the cover in which such document is mailed shall be deemed to be the date of delivery or the date of payment, as the case may be.

(b) This section shall apply only if the postmark date falls within the prescribed period, or on or before the prescribed date for the filing of the return, declaration, claim, statement, notice, protest or other document (determined with regard to any extension granted for such filing), or for making the payment (determined with regard to any extension granted for making such payment), and only if such document or payment was deposited in the mail, postage prepaid, properly addressed to the agency, officer or office with which such document is required to be filed or to which such payment is required to be made.

(c) If any return, declaration of estimated tax, claim, statement, notice, protest or other document or payment is sent by United States registered mail, such registration shall be prima facie evidence that such document or payment was delivered to the agency, officer or office to which addressed. To the extent that the Director shall prescribe, certified mail may be used in lieu of registered mail under this subsection.

(d) When the last day prescribed under the authority of the revenue laws of this State, including any extension of time, for performing any act falls on Saturday, Sunday or a legal holiday in this State, the performance of such act shall be considered timely if it is performed on the next succeeding day which is not a Saturday, Sunday or a legal holiday in this State.