For as long as we've been together -- over ten years now -- T and I have made it our custom to cook dinner and eat in for Valentine's Day. So after a quiet day at home, which included breakfast, reupholstering the downstairs cat tree, and watching "V for Vendetta" (which was pretty good, very style-over-substances, I really ought to read the graphic novel), we went to the store and picked up some steaks, cooked them up, and had a nice dinner together, nothing fancy, although we did pull out some candles. A low key celebration, as usual, but that's what works for us. It's been ten good years; with luck, we'll have ten more, and then another ten after that.
I hope everyone else had a lovely day as well, whether you celebrate or not. :)