INTERCALATION. By this name they call the Jewish custom of adding a thirteenth month to their Lunar year, at the end of every two or three years; that is to say, that in the revolution of the Cycle of nineteen years, there are seven of thirteen months each, and the rest are only of twelve months. When an intercalary month is added to the year, this month is placed between February and March, and then there is a first and second Adar; and the latter is called Ve-Adar, as if we should say, and again Adar, or Adar a second time.
The necessity of this Intercalation arises from their following the course of the moon in their year; whereas we follow the course of the sun. In the Solar year we intercalate one day every four years, in the month of February, and this fourth year is called Bissextile, because we reckon for two days together the sixth of the Calends of March, that is to say, the 24th and 25th of February; which, in the Bissextile years, instead of eight and twenty, has nine and twenty days. This Intercalation is founded on the six hours wanting eleven minutes, which the sun imploys in performing its course, beyond the 365 days which compose the common Solar year.
The Intercalation of the Jews, is owing on the contrary to all the Lunar months being not so long by twelve hours, or thereabouts, as the Solar months; which at the end of three years, makes nine and twenty or thirty days. See the article Months.