Admission offer deadline extension I received an admit (tuition fee waiver + TA) from a university for PhD in Physics. I was offered in the last month, and I replied the graduate admission director that I will inform him regarding 'accepting or declining' the offer around end of the march.
(the deadline for accepting the offer on April 15)

I thought I will get maximum decisions in March but few of the universities have put me on their waiting lists and probably they will decide by April 15.

Should I email the graduate admission  director  to extend the days? I mean, I will ask him that I inform him by April 15.

OR, as the deadline is 15th April, it is not necessary to email him to extend the date?
I think a very short email saying you are going to wait until the deadline for accepting the offer, with no further explanation, would be enough. You are not doing anything wrong and April 15th is the original deadline.

Perhaps it wouldn't even be necessary to explain anything. However my suggestion is that it is always better to be wrong on the well-mannered/ cautious side, than thinking "it is not necessary" and then find out it actually was.