Metonic cycle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A period of 235 lunar months, or about 19 years in the Julian calendar, at the end of which the phases of the moon recur in the same order and on the same days as in the preceding cycle.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy, calendar studies A particular approximate common multiple of the tropical year and the synodic month; in other words, the 19-year period over which the lunar phases occur on the same dates.