This Halacha is an excerpt from our Sefer
When do the three weeks begin; From the night of the 17th or the day?
Many Poskim rule that the laws of Aveilus begin from the night of the 17th of Tammuz. Other Poskim however rule that if it is a time of need one may be lenient on the laws of Aveilus at night until the morning. Practically, we are stringent, although regarding weddings one may be lenient in a time of need to have the Chuppah on the day of the 16th and then have the meal and dancing take place on the night of the 17th.
If the 17th falls on Shabbos: In a year that the fast fell on Shabbos and was hence was pushed off to Sunday all the laws of Aveilus begin starting from Motzei Shabbos according to all opinions. One may not be lenient even in a time of need.
Do we start from sunset of the 17th or from Tzeis Hakochavim?
Some Poskim rule that one is to be stringent to begin the laws of Aveilus starting from sunset.
Is one to diminish in happiness during the entire three weeks or only beginning from Rosh Chodesh Av?
List of customs that begin to apply from the 17th of Tammuz, as explained in this chapter:
- No Weddings
- No Haircuts
- No music or dancing
- No Shehechiyanu
- No buying or wearing expensive garments
- No hitting
- Avoid matters of danger
List of customs that only begin to apply during the nine days or during the week of Tisha B’av:
- Delaying a court case. [See Chapter 3 Halacha 2]
- Not to buy or sell expensive items of Simcha. [See Chapter 3 Halacha 3]
- Not to build or renovate for pleasure. [See Chapter 3 Halacha 5]
- Not to plant for purposes of pleasure. [See Chapter 3 Halacha 6]
- Not to eat meat or wine. [See Chapter 3 Halacha 8-9]
- Not to launder clothing or wear freshly laundered clothing. [See Chapter 3 Halacha 10]
- Not to purchase or wear any new clothing [See Chapter 3 Halacha 13]
- Not to bathe or shower. [See Chapter 3 Halacha 15]
- Not to cut nails. [See Chapter 3 Halacha 16]
The Customs applicable to Sephardim:
The Sephardic custom is to begin the following prohibitions only from the Nine days or the week of Tisha B’av:
- Weddings [nine days]
- Cutting hair [week of Tisha B’av]
- Buying cheap Clothing [Week of Tisha B’av]
 Chida Chaim Sheal 1:24; Ashel Avraham Butchacher 551; Tzitz Eliezer 10:26; Divrei Moshe 1:33; Shraga Hameir 2:13; Piskeiy Teshuvos 551:7
 Igros Moshe 1:168; 4:112; Even Haezer 1:166; See Shearim Hametzuyanim 122:1
 Piskeiy Teshuvos ibid; See there footnote 51 that even Rav Moshe Feinstein intended with his Teshuvah to simply allow the Chupah during the day and the meal at night and not to permit all the customs of Aveilus without limit.
 Igros Moshe 1:168
 Ashel Avraham Butchach 551; See M”A 551:26 regarding nine days
 Kaf Hachaim 551:4
 Implication of Poskim
 Yifei Lalev 5:2