Yes, it is a must.
Say, someone will be retired in May, 2015.
As 31st May is Sunday, (i.e. a Govt. Holiday), the retirement date will be 30th May.
Accordingly, in the release order, it is to be mentioned that, the date of retirement of such person is 31st May, 2015 which is a Govt. holiday, hence he is released on 30th May, 2015.
This is also applicable if the last day is a Govt. Holiday.
Yes. Rule 79 is related to this matter.
Rule 79 of WBSR:-
When a Government employee is required to retire, revert or cease to be on leave on attaining a specified age, the date on which he attains that age shall be reckoned as a non-working day, and the Government employee shall retire, revert or cease to be on leave as the case may be, with effect from and including that day.