Handling several acceptance letters from different schools
It would be polite to let them know that you "won't be ready to make a decision before some date". Asking them to hold the offer open for a bit and requesting a reply. 

If they really insist, then you have to make a decision about acceptance, but they might not, unless they have an overabundance of candidates. But, going past their due date implies risk that the acceptance will be withdrawn. So, be prepared to make a decision on short notice. 

Letting the date pass without notifying them is probably your worst option, however.