Rosh Chosdesh Adar - Using Simcha to come closer to Hashem.

From a Shiur by Rabbi Elieze Miller about Rosh Chodesh Adar from Naaleh

Every person has a bit of Amalek inside of himself and on Purim this is destroyed.  Therefore Purim is a special time for Teshuva.  The Belzer Rebbe said that if a person did not succeed in doing proper Teshuva in Elul , he has a second chance on Purim.  On Yom Kippur we do Teshuva through fasting, and on Purim we do Teshuva through happiness, but the purpose is the same, coming closer to Hashem.