The Laws of the Three Weeks (1)

1. Since the troubles of the destruction of the temple commenced on the seventeenth of Tamuz, the custom is that from that day onwards we start conducting ourselves like mourners for certain things. This continues until after the ninth of Av. It is correct for anyone who fears Hashem to recite Tikkun Chatzos each day just after midday. (Kitzur Shulchan Aruch 122)

2. One does not conduct a wedding in these days, but one may arrange an engagement, even with a meal, until the first day of the month of Av. From then onwards one may make an engagement with titbits but without a meal. (Latter authorities)