This is one of the "-mas" days Malory mentions occasionally, and until today I didn't know when it was. Apparently it falls on 2 February, "exactly 40 days after Christmas." The date has to do with a Jewish ritual of purification for new mothers and the first presentation of their children at the Temple in Jerusalem. This was to happen forty days after the birth, so this would be the date if we are right about the date of Christmas.
So that's something I didn't know until this morning.