When Tisha B’av falls on Sunday, when is considered the week of Tisha B’av?
When Tisha B’av falls on Sunday, the week beforehand is not considered the week of Tisha B’av, and there is hence no such concept of “the week of Tisha B’av” for that year. However when Tisha B’av falls on Shabbos and was pushed off to Sunday, it is disputed as to whether the entire week before is considered the week of Tisha B’av. According to all, however, this dispute is only with regards to those mourning customs that are forbidden due to the letter of law during the week of Tisha B’av, however those mourning customs which are merely a Minhag, are permitted according to all during this week.
Sephardic custom: The custom of the Sephardim is to follow the lenient opinion, and not consider the week beforehand as the week of Tisha B’av. [They hence permit during this week all the mourning customs which only take affect for the Sephardim on the week of Tisha B’av. These include: Doing laundry, wearing freshly laundered clothing, bathing. Meat and wine however is abstained from after Rosh Chodesh.] Even according to Sephardim one is not to get a haircut during this week.
Ashkenazi custom: The Ashkenazi custom is to be stringent to consider the entire week as the week of Tisha B’av regarding all matters that are forbidden during this week from the letter of the law. [Thus we are stringent regarding all matters with exception to the following: 1) Playing music for Parnasa; 2) Cutting nails on Erev Shabbos; 3) Eating meat by a Bris.] Those matters in which there is dispute as whether they are forbidden on the week of Tisha B’av, there is more room to be lenient in this week.
 Michaber 551:4
 Michaber ibid in first opinion; M”A 551:17 that this applies according to all [even according to the 2nd opinion in Michaber]; Tur in name of Smag; Biur Hagr”a; M”B 551:38; Kaf Hachaim 551:77
 Michaber ibid: Some Poskim [Rosh; Ran; Stam opinion of Michaber] rule it is not considered the week of Tisha B’av and there is hence no concept of the week of Tisha B’av that year. There is an opinion [Tur in name of Smag; 2nd opinion in Michaber] however that rules that the custom is to consider the entire week beforehand as the week of Tisha B’av, with exception to Thursday and Friday. [ibid] Practically, some Poskim rule like the former opinion, as so is the main ruling of the Michaber. [Shulchan Gavoa 551:27; Ben Ish Chaiy Devarim 12; Moed Lekol Chaiy 10:25; Yabia Omer 6:5; Yechaveh Daas 3:39] Others rule it is proper to suspect for the latter opinion. [Kaf Hachaim 551:78]
 Such as laundry
 Such as playing music for Parnasa; cutting nails on Erev Shabbos; eating meat by a Bris.
 M”A 551:35 as explained in Levushei Serud and P”M 551 A”A 35
 As rule the Shulchan Gavoa 551:27; Ben Ish Chaiy Devarim 12; Moed Lekol Chaiy 10:25, brought in Kaf Hachaim 551:78; Yabia Omer 6:5; Yechaveh Daas 3:39; However Kaf Hachaim ibid himself concludes to suspect for the stringent opinion.
 Ben Ish Chaiy Devarim 12; Moed Lekol Chaiy 10:25; Kaf Hachaim 551:78
The reason: In order to enter into the fast in an unorderly state. [ibid]
 Piskeiy Teshuvos 551:23, following the ruling of the M”A 551:35 as explained in Levushei Serud and P”M 551 A”A 35
 Shaareiy Teshuvah 551:1 in name of Elya Raba regarding a haircut for a Baal Bris