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)

Edward Caissie

Simple plugin that adds classes to the <code>body_class</code> output upon activation, including: a full list of date related classes; a few holiday classes; and, a generic sample full calendar extension.
<h4>Copyright 2012-2015 Edward Caissie (email : edward.caissie@gmail.com)</h4>
This program is free software; you can redistribute it and/or modify 
 it under the terms of the GNU General Public License version 2, 
 as published by the Free Software Foundation.
You may NOT assume that you can use any other version of the GPL.
This program is distributed in the hope that it will be useful, 
 but WITHOUT ANY WARRANTY; without even the implied warranty of 
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 
 GNU General Public License for more details.
You should have received a copy of the GNU General Public License 
 along with this program; if not, write to the Free Software 
 Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
The license for this software can also likely be found here: 
 http://www.gnu.org/licenses/gpl-2.0.html

BNS Body Classes

Simple plugin that adds classes to the body_class output upon activation, including a range of date related classes.

WP Scheduled Themes vs BNS Body Classes