I do have 1 but i don't work to much on this site and after check statistics i don't have anything like Donna, but site has great ranking only on M and Y, for G i don't have enough links.
I have a holiday site for most of the holidays, and yes, I have one for valentines day, so of course, I'll be watching it like a hawk this year. I think you are right about the commercial aspect of it. I don't imagine my groundhog day site will trigger the effect. lol.
I wanted to add something to this conversation that I just noticed -
About a year ago when I first found SEO sites and started to dig in I had an idea about a series of content pages to create. I looked at G's #1 serp result then created what I thought was slightly better layout/code/content to rank for the term I was going after.
I did. I hit #1 for this low competition word and started seeing traffic right away, and in the meantime this site I modeled by template after began to drop, to the point it was somewhere between 50-100. During that time (9-10 months) mine fluctuated, bouncing from #1 to #30, but usually in the top 10.
Well here we are, almost exactly a year later, and the site I first used as reference is back at #1. No code changes, nothing really new in BL.
Another thing I noticed when I went over the historical data patent.
There's a lot in there about season specific ranking so basically it could very well be season specific. I had some trouble connecting the dots in my previous post but the patent explains pretty nicely how it's done.