If this were not a leap year, then today, the 14th of Adar I would be Purim. In a leap year, however, an extra month of Adar is inserted so as to align the lunar calendar of Judaism with the solar calendar of the seasons(Otherwise, Jewish holidays would move around the year, instead of staying anchored in their accustomed times.)
Should Purim be celebrated and the Megillah read in the first or second month of Adar? The answer was not obvious to the rabbis. The Talmud (Megillah 6b) records a debate. Rabbi Eliezer asserted that it should be read in the first Adar. This makes a lot of sense. There is, after all, a principle that we should run to do a mitzvah at the earliest opportunity.
Rabbi Shimon Ben Gamliel disagreed, arguing that it should be read in the second month of Adar. It is more important to place the redemption of Purim close to the redemption of Pesach (which we celebrate in the month of Nisan that follows). The 14th of Adar I, however, remains a happy day even though Purim is deferred until next month.