Intercalary: Inserted out of the common order to preserve the equation of time, as the twenty-ninth of February in a leap year is an intercalary day. – A Dictionary of the English Language by Robert Gordon Latham, founded on that of

The year of the sun consisteth of three hundred and sixty-five days and six hours, wanting eleven minutes; which six hours omitted, will, in time, deprave the compute: and this was the occasion of bissextile, or leap year. – Brown, quoted by Johnson

For leap year comes naething but ance in the four. – Robert Shennan, “Leap Year”

It takes three springs to make one leap year. – The Comic Almanack