The Three Weeks span from the morning of the Fast of the 17th of Tammuz until midday of the day after the Fast of Tisha B’Av.
During the Three Weeks we may not perform or attend weddings under any circumstances, even if no festive meal is served. It is permitted to get engaged during this time, and a seudah (meal) may be served.
During the Three weeks it is forbidden to dance or play or listen to musical instruments. We may not buy or don new clothing. This does not apply to all types of clothing. For example, we may buy new socks, undergarments, shoes or shirts.
It is also forbidden to take a haircut or to shave, or even to just trim one’s beard. Someone who must shave for business reasons and has a hetter (dispensation ruling) from his Rabbi, may do so during the Three Weeks, but not during the Nine Days. It is permitted to trim a moustache that interferes with eating.
A Married woman may cut hair that protrudes from beneath her hair covering. She may also pluck her eyebrows and trim her eyelashes.
It is permitted to cut one’s nails during the Three Weeks.
It is permitted to comb your hair during the Three Weeks, even though it is possible that some hair may get torn out during the combing.
It is permitted, on the day of a bris milah, for the father, the mohel and the sandek (man who holds the baby during the bris) to take a haircut. But it is NOT permitted to take a haircut in honor of a Bar Mitzvah.
During the Three Weeks one should refrain from doing anything that might be dangerous in any way. This is a time when evil things have happened to the Jewish People, many times, over the past two and half thousand years. During the Nine Days especially, the statistics of fatal Jewish teen car accidents in the United States is way out of proportion with those of the rest of the year, to name the most notable example.
During the Three Weeks the Haftoros (passages from the Prophets) that we read on Shabbos (the Sabbath) are relevant to this period of time. These are the Three of Pur’oniyos (misfortune), wherein the Prophets Jeremiah and Isaiah warn of the impending Destruction of the Holy Temple and our exiling to Babylon. They are part of the general tone of the Three Weeks, and it is the only concession that Shabbos makes to that sad tone.
This is a sad time, which is difficult for a religion and people that live so much in joy. But just as serving Hashem with joy is an obligation the entire year, keeping a measure of sadness during this time is also our obligation, and is, during the Three Weeks, how we must serve Hashem.
But Hashem will overturn our sadness to joy, when he takes us out of exile and restores to us our judges, courts, our land, the city of Jerusalem, and His Service in the Holy Temple. May it happen soon!