On the 13th of Adar the Jews gathered to defend their lives and take vengeance on their enemies. They had to plead to Hashem for mercy that he come to...

Because in Parsha Ki Sisa where it talks about giving the half shekel it says Terumah 3 times.

One gives tzedaka in the costs of the coins picks it up and gives it back so the next person can do it since the half coins are hard to come by.

The months of Adar since this is the months Hashem performs miracles and we have salvation.