Ides

Þa monðas þe habbað iiii nonas æfter kalendas... habbað to idus xiii dagas and to ii kalendas eahtatyne. Those months that have 4 nones after the kalends... have 13 days to the ides and eighteen to the second kalends.

The Roman Month its several days divides By reckoning backwards, Calends, Nones, and Ides.

For the modern reader of Latin the most irritating pecularity of this system of dating is that the days after the Ides of any month carry the name of the following month... Another trap for the unwary lies in the fact that the Roman calendars given in most reference books are Julian, not pre-Julian. When Caesar added ten days to the Roman year he put them near the ends of the seven 29-day months, one or two in each. As a result, instead of the day after the Ides of all months being a.d. XVII Kal., in these seven months it is either a.d. XVIII Kal. or a.d. XIX Kal., and all the following days change correspondingly.

[March, May, Quintilis, and October] also have their Nones on the seventh, as Numa ordained, because Julius changed nothing about them. As for January, Sextilis, and December, they still have their Nones on the fifth, though they began to have thirty-one days after Caesar added two days to each, and it is nineteen days from their Ides to the following Kalends, because in adding the two days Caesar did not want to insert them before either the Nones or the Ides, lest an unprecedented postponement mar religious observance associated with the Nones or Ides themselves, which have a fixed date.

The third day before the ides of March is March 13th; the third ides of August is August 11th; and the third of the ides of November is November 11th.