On this date, in the year following the Holy Temple’s destruction, G‑d tells Ezekiel to take up a lamentation for Pharaoh, king of Egypt, and for the Jewish nation’s other enemies, foretelling their ultimate downfall.
Read the prophecy here: Ezekiel ch. 32
In regular years, the 15th of Adar is Shushan Purim, the festival that celebrates -- in Jerusalem and other ancient walled cities -- the salvation of the Jewish people from Haman's evil decree in the year 3405 from creation (356 BCE). In a leap year -- which has two Adars -- Shushan Purim is observed in Adar II, and the 15th of Adar I is designated as Shushan Purim Kattan, the "Minor Shushan Purim."
There are no special observances associated with Shushan Purim Kattan, other than the omission of Tachnun ("supplications") from the daily prayers and a prohibition against fasting or holding eulogies on this day. The Code of Jewish Law cites an opinion that one should increase in festivity and joy, but rules that there is no obligation to do so; "Nevertheless,a person should increase somewhat in festivity... for 'One who is of good heart is festive always' " (Shulchan Aruch, Orach Chaim 697:1).