Etymology
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intercalate (v.)
"to insert a day into the calendar," 1610s, from Latin intercalatus, past participle of intercalare "to proclaim the insertion of an intercalary day," from inter "between" (see inter-) + calare "to call" (an intercalary day; see calendar). Sometimes used in a general sense, "to insert between others" (1824). Related: Intercalated; intercalating.

A necessary process in the Roman calendar to balance the solar and lunar aspects of it. Intercalation was done after Feb. 23 or 24 (the terminalia), every two or four years. Twenty-seven days were intercalated, making a full intercalary month (which included the last four or five days of Februarius), known as mensis intercalaris (and also known, according to Plutarch, as Mercedonius). No one now knows why the intercalation was done in the middle of February rather than after its end, unless it was because the important festivals at the end of that month (Regifugium and Equirra) were closely associated with holidays in early March. After Caesar's reform (46 B.C.E.) the only intercalary day is Feb. 29 every four years.
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intercalary (adj.)
"inserted into the calendar," 1610s, from Latin intercalarius "intercalary, of an intercalary month," from intercalare "proclaim an intercalary day" (see intercalate). General sense of "interpolated" is attested from 1798.
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intercalation (n.)
1570s, from Latin intercalationem (nominative intercalatio) "insertion of an intercalary day," noun of action from past participle stem of intercalare "proclaim an intercalary day" (see intercalate). The general sense "insertion of any addition into an existing series" is from 1640s.
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epact (n.)
1550s, "a number attached to a year to show the number of days into the calendar moon on which the solar year begins;" 1580s, "number of days by which the solar year exceeds a lunar one of 12 moons;" from French épacte (12c.), from Late Latin epacta "an intercalary day," from Greek epakte (plural epaktai, in epaktai hemerai "intercalary days"), from fem. of epaktos "brought in, imported, alien," verbal adjective of epagein "to add, bring forward," also "intercalate," from epi "on" (see epi-) + agein "put in motion, move; push forward, advance," from PIE root *ag- "to drive, draw out or forth, move." Related: Epactal.
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