(historical, often capitalized) The notional full-moon day of a Roman month, occurring on the 15th day of the four original 31-day months (March, May, Quintilis or July, and October) and on the 13th day of all other months.
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.
[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.