leap year: [14] The inspiration for the term leap year is probably simply that in such a year the day on which any given date falls ‘jumps’ one day ahead of where it would have been in an ordinary year. The metaphorical application of the notion of ‘jumping’ to this phenomenon predates the first record of the term leap year: medieval Latin, for instance, used the term saltus lunae ‘moon’s jump’ for the nineteen-yearly omission of a day from the lunar calendar, and this was translated into Old English as mōnan hlyp ‘moon’s leap’.
late 14c., from leap (v.) + year. So called from its causing fixed festival days, which normally advance one weekday per year, to "leap" ahead one day in the week.
