I seem to remember it being something about the middle day of the month. There are other days which are the first and last days (and I guess the nones as well from OAE's post). Anyone have a better Latin lesson memory on this?
And did you know that tomorrow will be 'a.d.xvi Kal.Apr.' - aka Dies Sancti Patricii - where 'a.d.' means 'ante diem', not 'anno domini' ? And you thought the Yanks with their 9/11 had their Kalendarium arsewise!