(RabbiKaganoff.com)
The Mishnah teaches that “Mishenichnas Av mema’atim b’simcha,” “Once Av enters, we decrease our happiness” (Taanis 26b). Although the Mishnah does not clarify what we must do to decrease our happiness, the Gemara (Yevamos 43a) lists four activities that are banned during these days:
(1) We decrease our business activities.
(2) We refrain from construction and planting intended for joyous reasons (Yerushalmi Taanis, cited by Tosafos to Yevamos 43a s.v. Milisa).
(3) We do not conduct weddings.
(4) We do not make a festive meal to celebrate an engagement. (This interpretation of the Gemara Yevamos follows the Ramban in Toras Ha’adam and the Tur Orach Chayim 551, but is not the approach used byRashi ad loc.)