Did you go through the URTA program for applying to grad school, or did you apply to specific schools? In my experience the time frame differed for each school and whether I interviewed through URTA's, or had applied to the school directly.
Some schools had certified mail delivered on the very day they said was the first day they would make offers; other schools sent emails a few weeks after the offers date had passed.
I would say that if you've been offered a spot at one school and are waiting to hear from your top choice, it's always okay to politely inquire about your status.
Hope that helps!