Rosh Chodesh Adar Alef (the current Hebrew month) celebrates Purim Katan (mini Purim). We don't really observe much on this day, so what is it all about? The Stichiner Rebbe offers two suggestions:
1) Katan = a minor. A minor observes mitzvos on the level of אינו מצווה ועושה (volunteer basis). So too this "optional" Purim is a day when we optionally opt in to think about the lessons of Purim.
2) Katan = a minor. A minor keeps mitzvos before their coming of age so that they can prepare for the mitzvos once they pass 12 or 13. In the same way, this Purim Katan is a preparation for the Purim that will come in the second Adar.
Rabbi Shlomo Einhorn