How to send end-of-the-year wishes in academic environment?
There are many options which can be made by themselves or in some combination. 


You can design a "card" to attach to the mail which could be informative or aesthetically pleasing (but do not make it a huge file).
You can, as I and my former adviser do, make a "Christmas letter" that summarizes the year both academically and private. You need to adapt to your audience here since the private section should not be too detailed if your are not really close friends, but comments on travels with a nice shot is never a bad idea.
You can divide the send list up into different categories but the backside of this is that if you group people and group them wrong (from their perspective, it can become embarrassing). 
The previous point is also true for listing people in some order; so alphabetical, even in the way you list the e-mail addresses, is probably wise.


Do not emphasize Christmas and New Year per se since not everyone may be celebrating these primarily Western holidays. Use something more neutral formulation such as "Best Holiday Greetings" or "Best Wishes for the Holiday Season". Again, know your audience and adjust.

You can send a general more formal greeting to a larger group and then follow up with a more personal greeting to specific people you wish to share more personal greetings.


So, think about the options and combine to taste.