Not sure about Switchvox, but using the FreePBX GUI, I set up a time group called Holdays. At the first of the year, go in and define each of the holidays. Create a generic IVR or announcement that tells the caller you are closed in observance of the holiday. Route calls to the Holiday Time condition first. If it is a holiday, the caller gets the message, if not the call proceeds normally. This just has to be setup once a year.
I assume Switchvox has a similar setup.