proemptosis

from The Century Dictionary.

• noun In chronology, an anticipation, or occurrence of a natural event sooner than the time given by a rule; especially, the falling of the new moon earlier than the nineteen-year period would make it, amounting to one day in 312½ years according to Clavius and the constructors of the Gregorian calendar (really 310 years), in consequence of which a lunar correction is introduced into the tables for calculating Easter; also, the effect of the precession of the equinoxes in making these come before the sun has performed his circuit among the stars. See metemptosis.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Chron.), rare The addition of a day to the lunar calendar.

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

• noun The addition of a day to the lunar calendar.