This coming Wednesday, the fourteenth day of Adar Aleph, is Purim Katan. The source of this day dates back to the days when the calendar was set by testimony of the new moon. The decision to make a leap year was not decides many times until the end of Adar. By this time, Purim had already been celebrated. When the second Adar was added, Purim would be celebrated again. Today, when the calendar is preset, it would seem that the first Adar would be insignificant in regards to Purim. However, we have added this day to our calendar as a commemoration of the way things were during the days of the Beis Hamikdash.
Being that the source of Purim Katan is only a custom, it would get overridden against something with a stronger halachic source. For example, if someone took a vow fast on this day, this vow would be binding. (Mishna Berura 697:3) The customs observed on Purim Katan is not to recite tachanun, give eulogies or fast. (These are also observed on the fifteenth of Adar, the day of Purim in the walled cities.) We should also have a small seuda.

Rabbi Tzvi Aryeh Hyman, Mir Yeshiva
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