with a cron job.
First re name folder 1 to Folder X
Second rename Folder 2 to Folder 1
When done rename Folder X to Folder 1
or use php with a Else IF useing the dates
to read a include file, for what dates you have.
Or a smart arse may use a css file for the change the theme
but can take time to get used too.
With the cruise wirral christmas headers they kick in
with php and a include file
Hope that helps