As we have previously explained (see here), on the 17th of Tamuz of the year 68 of the CE, after many months of siege, the walls of the city of Yerushalaim were destroyed by the Roman legions.
During three week the invading army pillaged the holy city, and thousands of Jews were tortured, killed or taken as slaves. The second Bet haMiqdash was destroyed and burned three weeks after the Romans enter the city: the 9th of Ab. More than 600 years earlier, --in 586 BCE-- the first Bet haMiqdash was destroyed also on the 9th of Ab.
As we approach the 9th of Ab --the National day of Mourning for the Jewish People-- we observe certain restrictions associated with mourning. The restrictions are stricter as we get closer to the 9th of Ab. These customs differ from community to community.
A few illustrations:
WEDDINGS: The Rabbis of the Talmud (and Maimonides) did not mention any restrictions to be applied before the beginning of the month of Ab. For Sephardim it is not forbidden to have a wedding ceremony between the 17 of Tamuz and the beginning of the month of Ab. The Ashkenazi custom however, is to avoid wedding ceremonies from the 17th of Tamuz. However, and although technically permitted, the practical custom in our days is that Sephardic Jews do not celebrate weddings during the three weeks.
SHEHEHEYANU: The Shulhan Arukh mentions that it is good to refrains ourselves from eating a new fruit that will require the recitation of the blessing Sheheheyanu during the three weeks. The custom in this case is to allow the recitation of Sheheheyanu in Shabbat (for Sephardim and Ashkenazim, penine halakha).
HAIRCUT: For Sephardim it is permitted to get a haircut or shave until the week of Tisha beAb. The Ashkenazi tradition (and of some Moroccan Jews as well) is different: haircut or shaving is forbidden from the 17 of Tamuz. (Haircut restrictions do not apply to women).
More on the customs of
The Three Weeks
by Rabbi Shraga Simmons,