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)

David Nickles

Show Date for Holiday’s Name quickly calculates and shows the full date of major U.S. holidays for the current calendar year. Add the full date notation for many U.S. Federal holidays, Hawaiian state holidays, and various cultural observances in your posts easily with just the name of the day. For federal and state public holidays that may land on a weekend, you can choose to display the actual date of the holiday or the designated observed weekday date.

Show Full Date for a Day’s Name

Add the full calendar date for many U.S. and Hawaiian holidays in your posts easily with just the name of the day.

WP Scheduled Themes vs Show Full Date for a Day’s Name