I went through this process a couple years ago (one of the first SF match years if not the first) and the invites tended to come in chunks.
I did 12 interviews and these were the respective dates:
2 invites in December (Midwest, South)
2 invites in January (West Coast)
6 in February (2 NYC, 1 west coast, 2 boston, 1 midwest)
1 in April (Boston)
1 was my home institution so there was no relevant date.
I started receiving contact from programs around December about supplemental documents (all of the stuff that was already in my SF match application) so I kept a generic email filled with these that I just forwarded and rewrote to each place that asked. January is when I received the most supplemental documents requests and usually I was sent an interview invitation within a week or so of me having sent the document.