Adam Erstelle

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

WP Scheduled Themes

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

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 Themes vs Simply Snow