netmeg - 6:47 pm on May 13, 2013 (gmt 0) [edited by: netmeg at 6:49 pm (utc) on May 13, 2013]
Seasonal holidays are my thing. Google has been doing that for years. I have a spreadsheet with almost daily rankings for a single keyword going back to 2006 (most of you who know me will guess what it is) and the movement up and down as the season comes and goes is marked. Over a given 12 month period, the keyword will rank as highly as #3 and as low as #96, and then back again. Note: the keyword is very broad, and not one I would expect to rank for, to be honest) I assume it rises because Google recognizes the increase in searches related to the season, and is making its own best guess as to user intent.
You can also notice the differences when personalization and better geotargeting started kicking in.
So what you're seeing now, I've been seeing for a long time. I don't think it's new.
[edited by: netmeg at 6:49 pm (utc) on May 13, 2013]