On 28th, we had our first snow in my town, and my blog has snow falling from today (^o^). Last year, it began from 28th of December though, I let it from 1st of December this year. I think the date or the time depend on the server timezone, so you might see the snow even if you are still on the last day of November your local time. Please accept it (^_^;).
This is one of Jetpack plugin features, and I think the original script is Snowstorm by Scott Schiller.