what can we do while waiting for the decision date?

no need to write to them

it'll only slow the whole process

unless you have something very solid that can dramatically change your candidacy

or you're only annoying them

the admission has nothing to do with the letters you writing to them

it all depends on the application you submitted and your interview

only when they ask for additional information, then you just provide the needed materials

then there's nothing else you need to do

it's all done in your application package

if you want to do something, do it before you submit