The person I spoke to was being very vague, apparently they have already sent out the first batch of letters last friday. I asked him if they do it by area, but he said no, it didn't matter. He told me they need time to fold the envelopes and get everything stamped, that's why it's going to take a while for some people and we should receive them in the next two weeks (again, very general), but there is a possibility that we might get it this week. (That's just mean, teasing but not confirming..)
I'm guessing the window you are referring to is "waitlist"? Because according to the person, you should have received a response of some kind by the second week of April.