"One year, most of Adar went by and it didn't rain. They sent for Choni the Circle Maker. He prayed and the rains didn't come. He drew a circle, stood in it and said: 'Master of The World! Your children have turned to me; I swear in Your great name that I won't move from here until You have pity on Your children.' The rains came down." (Talmud, Taanit 23a)

Adar 20 is the yahrtzeit (anniversary of the passing) of Rabbi Yoel Sirkes (1560?-1640), Rabbi of Krakow and author of the Bayit Chadash ("Bach") commentary on the great Halachic work, the Arba'ah Turim.