To enable you to play with the holiday event enabled, we've created a dedicated addon to allow you to apply this to your server. 

You'll find this under Edit Profile > Addons :

Holiday Event

Simply click the blue add button to add it to the currently installed addons list and then click the Save & Apply button. 

This will then add the -holidayevent command line parameter to your startup options to enable this on your server. 

Happy Holidays!