Adam Erstelle

This plugin allows a wordpress administrator to schedule an additional css file to display on the website for holidays or special events for all visitors.
The css file will be added to the live site. Great for setting a Christmas theme!
If you just want to have a theme loaded instead of just a css file, check out the wp-scheduled-themes plugin.

WP Scheduled Styles

Schedule a css file to display on the live site for holidays or special events. (Available from Appearance - Scheduled Styles)

Justin Kopepasah

There are a lot of plugins that create a snowing effect on your site, but I wanted one that simply added a snowing effect without options, clutter or drastically affecting my sites speed.
No options. 
No bloat. 
Simply Snow.

Simply Snow

A WordPress plugin that, when activated, will add a snowing effect on your site.

WP Scheduled Styles vs Simply Snow