The term leap year, however, isn’t recorded in English usage until the late Middle English period. From John de Trevisa’s translation of Polychronicon Ranulphi Higden, 1387:
That tyme Iulius amended the kalender, and fonde the cause of the lepe yere.
As Zythophile mentions, monan hlyp is found in Old English. It appears in AElfric’s De Temporibus Anni, from c.993:
se dæg is gehâten Saltus lune • þæt is ðæs monan hlyp
(the day is called Saltus lune, that is the leap of the moon)
(The OED has this cite, but calls it the Saxon Leechbook, which is a later name for a collection of Anglo-Saxon science texts that includes this work by AElfric.)
And the Old Norse hlaup-ár (leap year) is even older, but AElfric didn’t use that term and his work was the chief Old English one on the subject. The term is also missing from the Old English translation of Bede, who was the ultimate medieval authority on all things calendrical. I’d have to check the other major Old English work, Byrhtferth’s Enchiridion to see if it’s used there, but the OED doesn’t cite it and Byrhtferth mainly copied AElfric, so I don’t think it’s likely to be found there.
(My project for last semester was a translation of De Temporibus Anni, so I’m up on all the source material.)