Change theme on a specific date, ex.: have a christmas theme for december and revert back to normal after.
The admin panel will allow to move re-order your theme changes. Just remember that, the top is the lowest and the bottom is the highest level. So, if you have a theme change ('theme1') set for the 23/07 - 01/08 first and theme change ('theme2') for the 24/07 - 26/07, and today's date is 24/07, 'theme2' will be displayed unless you move down 'theme1'."
The rules, which you have created, will stay year on year.