Payments made by mail shall be deemed to have been made timely if postmarked not later than midnight of the due date. If the due date falls on a Saturday, Sunday or a legal holiday, payment shall be deemed to have been made timely if postmarked not later than midnight of the next business day.
Due date for payment in case of a redetermination shall be determined in the same manner as for the original determination.
12- 172 C.M.R. ch. 2, § 5