Almost all physics departments host a weekly colloquium, at which an outside speaker presents a talk on a current research topic -- it's traditional to take the speaker out to dinner afterwards. When I began my first appointment as a junior faculty member (at Ohio State), this task often fell to the single faculty members -- our schedules were more open, and we were grateful for the free meals and the company.
One Tuesday evening, a group of five of us (all guys) took the speaker out to a local restaurant, only to discover that they didn't have a free table. This was a bit odd for a Tuesday night, but we simply went to our second-choice restaurant, only to discover that it was full as well. Finally, one of us realized that it was Valentine's Day! That's right, none of us, including the speaker, had remembered that Feb. 14 was of any significance. (This story has a happy ending -- we found a Chinese restaurant with available seating).
My point is that for unattached guys, Valentine's Day is effectively invisible -- it's like Arbor Day -- it comes and goes without any notice whatsoever. (Did you remember to plant a tree on Arbor Day?) And then suddenly, and without warning, Valentine's Day becomes a Very Important Holiday, and one that You Had Better Not Forget. And depending on one's dating history, this transformation of Valentine's Day can cycle from invisible to momentous and then back to invisible again many times.
Of course, the real St. Valentine was an early Christian martyr who was beheaded. Try finding that in the greeting card aisle.