epact

n.
The period of time necessary to bring the solar calendar into harmony with the lunar calendar.

[French épacte, from Late Latin epacta, from Greek epaktē (hēmera), intercalary (day), feminine of epaktos, brought in, inserted, from epagein, to bring in, introduce : ep-, epi-, epi- + agein, to lead.]

time The (number of) days by which a full solar year exceeds a year of 12 full lunar months, else, particularly in calculations relating to Easter, the age of the Moon (i.e. the number of days after the new moon) at the start of the calendrical solar year. More generally, any length of intercalary days.

a period added to harmonize the lunar with the solar calendar