As a young boy, The Kotzker Rebbe was jumping on his bed in celebration of Rosh Chodesh Adar.
His father walks in and says: What’s all the noise?
Kotzker: משנכנס אדר מרבין בשמחה
Father: Then why did you stop jumping?
Kotzker: משנכנס אב ממעטין בשמחה