We've just welcomed the Hebrew month (new moon) of Adar - you may have noticed this because of the Chinese New Year's is also welcomed on a new moon. Other than this coincidence, the moon does not have quite the celebrated status in the west is it has on the Jewish calendar. But this was not always so. The word "month" is really the non-word "moonth." A "moonth" is a full moon cycle - from new to full to fully waned.
Here comes some math. A moonth is 29-30 days long. So a year of moonths - 12 lunar months - is 354 days long, whereas a solar year is 365 days long. This is the reason for the constant shift of Jewish holidays relative to the western calendar. A Jewish (lunar) year of 12 moonths is 11 days shorter than a solar year. But our holiday cycle is agricultural! We need our spring festival (Passover) to come in the spring. We need our fall holidays to arrive around the fall equinox. How is this done? Jews have a luni-solar calendar; we have a leap month 7 times in a 19 year cycle, roughly every third year.
Sensibly enough, the leap month is inserted in the late winter - before the spring planting season - so that the lunar and solar years synch up. 2016 is a leap year for us and so the month of Adar we have just welcomed is Adar One. Adar Two will begin on the next new moon and Purim will be celebrated on the full moon of Adar Two, March 23 this year. All the holidays will be a little "late" this year. Mystery solved.
One more note: The Hebrew calendar (and other aspects of our tradition) are fully integrated with astrological systems. This "double moonth" of Adar equals Pisces, the twin fish. It makes sense that our double month would be inserted during the reign of these twins. Purim itself - the story of reversal of fortunes - fits beautifully with the theme of these twin fish, swimming to and fro in a frothy mix. Now ya' know.