This month, Adar l, is an added month in the Jewish leap year. This year is a shanah meuberet (lit., “a pregnant year”), more commonly known as a leap year, on the Jewish calendar.
The Jewish leap year, which occurs seven times in a 19-year cycle, has 13 months instead of the regular year’s 12. This is so that the lunar-based Jewish year (which is 354.37 days) should remain aligned with the solar year (365.25 days) and seasons.
It is important to keep the calendars aligned in order for the festivals to retain their positions relative to the seasons as prescribed by the Torah. The added month is called Adar I, and is inserted before the month of Adar (termed Adar II in leap years).
Adar is the official “happy month,” as is written: “As soon as Adar begins, increase in joy!” In a leap year, we have two months of extra happiness!
The festival of Purim, celebrated on Adar 14, is in Adar II in leap years, while the 14th of Adar I is marked symbolically as Purim Katan—Minor Purim.