i applied 2 seasons ago, so my memory may be a bit faded. i remember setting up most of my interviews throughout october and into mid november. i don't know if the timing of the invites has anything to do with the "tier" of the program. remember that real people are looking through every application, and that takes time. most places seem to interview 10 times the number of positons that they have, and some may send out more invites in november as applicants cancel interviews, so more invites go out because program directors probably know the interviewees to positions ratio necessary to ensure they fill.