Only appears when there is a leap year, such as in 2015/16 and now 2018/19.
Purim itself is celebrated in Adar II. The next time this will happen is the Jewish year 5779, on February 19, 2019.
Adar I and II occur during February–March on the Gregorian calendar. Based on a line in the Mishnah declaring that Purim must be celebrated in Adar II in a leap year (Megillah 1:4), Adar I is considered the “extra” month.
Numbers 28:11-13 Tree of Life Version (TLV)
Rosh Chodesh: New Moon
11 “On the first of the month you are to present to Adonai a burnt offering of two young bulls, one ram, and seven flawless male lambs a year old, 12 with three tenths of an ephah of fine flour mixed with oil as a grain offering with each bull, and two tenths of an ephah of fine flour mixed with oil as a grain offering with the ram, 13 and with each lamb a tenth of an ephah of fine flour mixed with oil for a grain offering, a burnt offering as a pleasing aroma, an offering by fire to Adonai.
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