"Then our mouth was filled with laughter, and our tongue with singing" (Psalm 126:2). Rabbi Yochanan said that Rabbi Shimon ben Yochai taught that a person is forbidden to fill his mouth with laughter in this world, as it is said, "Then our mouth was filled with laughter, and our tongue with singing." When is this allowed? When the nations say, "The Lord has done great things for them."
It was said of Rabbi Shimon ben Lakish that all his days he never filled his mouth with laughter after hearing the words of Rabbi Yochanan. Rav Yirmeya was sitting in front of Rav Zeira and saw that he was sad. He said to him, "It is written (Proverbs 14:23), 'In all toil there is profit.'" Rav Zeira replied, "I am worried about Tefillin."
Mar the son of Ravina made a celebration for the rabbis, and brought a chair made of 400 Zuz worth of Caratha wood and a deer was roasted before them, and they rejoiced. Rav Ashi made a celebration for the rabbis, and brought a chair made of white glass and a deer was roasted before them, and they rejoiced. From this we learn that it is said, "Then our mouth was filled with laughter, and our tongue with singing."