Sentence:  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 <WSD>ides</WSD> and eighteen to the second kalends. 
Meaning of the word marked with <WSD> tags:  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.