Tisha B\’Av, the day of mourning in commemoration of the destruction of the two Temples, is notable for at least two reasons. For one, it may be the only holiday that Hallmark hasn\’t designed a card for. And it seems to be the one holiday that most Jews have heard of, but few seem to know much about. As with quarks and RNA and Rothko, we can drop \”Tisha B\’Av\” into a conversation, hoping all the while that we won\’t be asked to actually explain it.