if memory served me right, U Michigan sent out its first wave of interviews in early October even before all the LORs were complete. this was an anomaly. then there was a LONG draught when I thought I'd never hear from another school ever again. most of the interview invites (either positive or negative) were made right before Thanksgiving through Christmas. the schools I did not hear from by Christmas almost all ended up rejecting me.
good luck to all who're applying this year! by the way, it is highly advisable to reserve an interview spot ASAP when invited because they fill up quickly.