I would predict it to begin next Wednesday, but could easily see it being released the week after or even later today.

The best possible thing they could do for winterfest would likely be a new and permanent ice themed level. We have levels that appear for every monster family, but not for every status... so why not make an area based around ice in general, and then stick that in the game? Something like the jelly farm levels, but for ice areas would be sweet and wintery.