Section 7503, in part, says:
When the last day prescribed under authority of the internal revenue laws for performing any act falls on Saturday, Sunday, or a legal holiday, 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.
and
Section 401(a)(9) says, among other things, this:
(C)Required beginning date.—For purposes of this paragraph—
(i)In general.—The term “required beginning date” means April 1 of the calendar year following the later of—
(I) the calendar year in which the employee attains age 70½, or
(II) the calendar year in which the employee retires.
Thank You.
Please note the statutory language "...shall be considered timely" in Section 401. Not much wiggle room there, IMO.