The first time I listed this item, I didn't even think to use Valentine's Day as a keyword. Then when I was renewing the listing, I realized I was missing an opportunity. I bought the t-shirt because, let's face it, you just don't see very many unique and fun politically (or a-politcally?) themed tees out there, and I thought this was a great one.
For me, the moral of the story is: it's OK to think about holidays in offbeat ways, and always think creatively when it comes to the keywords for your listings.