Adar
Archimandrite Nikifor’s Biblical Encyclopedia
(beautiful, glorious) (Ezra 6:15; Esther 3:7, 13) — the sixth month of the civil, and twelfth of the sacred, Hebrew year, corresponding to the latter part of our February and the first part of March. In this month the well-known Jewish feast of Purim was celebrated. Every two years this month was repeated in the third year — that is, it occurred twice in that year, and on that occasion the feast of Purim was likewise celebrated twice, the second time with special splendor. The reason for the repetition of the month Adar every third year was as follows: since twelve lunar months make up only 354 days and 6 hours, the Jewish year was shorter than the Roman year by twelve days.
To make up the deficit relative to the Roman calendar, the Hebrews introduced into their calendar a thirteenth month, which they called Veadar, or the second Adar, by which the lunar year was brought level with the solar year.