In the same way that it is a Mitzvah to rejoice with the happiness of performing a good deed, so conversely it is forbidden to be happy for other reasons, as it says in the holy Zohar that it is forbidden to look at something which makes one happy for the smoke of the burning of our holy temple is an eye-cover for us. The Talmud also says that it is forbidden for a person to fill his mouth with laughter in this world. All the more so is it true that those who rejoice with the idol-worshippers on the days of their festivals or who rejoice and fill their mouths with laughter when they hear the voice of a fool with an abundance of words of mockery and foolishness are considered to be sinners and people who also cause others to sin, as we know that a person who is indulging in foolishness and sees that others laugh at his witticisms will feel his heart “expanding” and will add sins to his iniquities with all sorts of evil things. Chazal also say that people who do these things will not move from their places before a bad decree has been decreed upon them (G-d forbid) as instead of protesting about the honor of heaven they are indulging in laughter and light-headedness. Fortunate is the man who has not walked… and has not sat in a sitting of mockers. We should only rejoice and be happy on the day that Hashem made to keep his statutes and commandments.
ו׳ בתשרי ה׳תשפ״ב
"Moshe corrected them to Israel, to be asks and demands on the day's issue, laws of Pesach on Passover, laws of Atzeret at the assembly, the laws of festival on the festival" (Megillah 32) 1