In regular years, the 15th of Adar is Shushan Purim, the festival which is celebrated in Jerusalem and other ‘walled cities’.
In a leap year there are two months of Adar – Adar Alef (the First Adar) and Adar Bet (the Second Adar). So Shushan Purim is celebrated in Adar Bet, and the 15th of Adar Alef is Shushan Purim Kattan – the “Little Shushan Purim Kattan”.
“Tachnun” (“supplications”) and “Vidduy” are omitted from the daily prayers on this day.
There are no special observances associated with Shushan Purim Kattan.
However, Rabbi Moshe Isserles (Rema) notes that there are opinions that even the “Little Purim” should be marked by an increased joy. He adds that it is good to arrange a more plentiful meal than usual in the honour of Purim Katan.