Holiday Swag Bucks are earned, spent, and found primarily through the "Search" feature of Swag Bucks (around Christmas there was a holiday 5 which could only be earned by watching SBTV.) In case you forgot, "Search" can be found through Earn -> Search or it is also located directly on the Swag Bucks toolbar. Holiday bucks exist in predetermined denominations that are announced by the site at the beginning of the promotion. They can only be found around holidays.
There are always five holiday Swag Bucks to be found. If you acquire each one of these at least once you will earn a bonus of 50 Swag Bucks at the end of the promotion! Right now there are Valentines bucks. They exist in these denominations: 7,8,12,14, and 20. I like holiday bucks because not only does it add a little excitement to the search, but they are also adorable. Here is an example of the Valentine's Day 14:
Cute, right? Anyway if you're ever curious about what they look like, who is winning them, etc. you can always go the the Swag Bucks facebook page where some people literally troll the site all day talking about such things. If I have extra time on facebook, I'd rather spend it on the Free Samples site, but that's just me =)