Today is Purim Katan, or Little Purim.
In a Jewish leap year, when an extra month of Adar is added, Purim itself is celebrated in the second Adar, always one month before Passover. The equivalent date in the first Adar, the date which would be Purim were this not a leap year, is marked as a mini-holiday known as Little Purim. There are no obligatory festivities, though some do make the effort to have a modest festive meal.
Before the Jewish calendar was fixed by computation, the Sanhedrin would meet each spring to determine whether to add an extra month of Adar to the current year. If the decision was reached after Purim had already been celebrated, Purim would be celebrated a second time in the second Adar. So in ancient times, today might actually have been celebrated as Purim. Then, after the Sanhedrin announced the leap year, we would have had to prepare for another Purim, the real one.
You think that's a pain, wait till you hear about Pesach Sheni!