I also think it's a really good idea to let your clients know -- whether holiday time, etc. -- if you are not going to be available either by phone or email. In my experience I have not had to work with clients over holidays, with the exception of 'overflow' transcription work I take on now and then for other transcribers or for transcription companies.
It's a good idea also to ask one or two VAs who you know and trust if they would be willing to back you up should you need to take a few days off or in a time of an emergency -- just in case you have a client who needs and/or depends entirely on you to manage his/her business.