One may not perform several actions during the week in which Tisha B’Av falls. This is referred to as shavua she’chal bo. For example, one may not take a haircut or wash his clothing (Ashkenazi Jews are forbidden in these actions prior to the week of Tisha B’Av in accordance with the ruling of the Ramah). The Mechaber (Shulchan Aruch Orach Chaim 551: 4) writes that in a year when Tisha B’Av falls on Shabbos and is pushed off to Sunday (as it does this year) there is a machlokes as to whether there are any prohibitions during the week before Tisha B’Av. The Mechaber seemingly sides with the view that there are no halachos of shavua she’chal bo in such circumstances.
Many Achronim explain that the dispute is based on the understanding behind the establishment of the fast of Tisha B’Av. The Gemara in Ta’anis 29a says that the Beis HaMikdash was lit close to the end of the ninth day of Av and continued burning throughout the tenth day of Av. Reb Yochanan said, “Had I been in the generation when Tisha B’Av was established, I would have established it on the tenth day of the month since the majority of the Beis HaMikdash burnt on that day.” The Gemara says that the Rabbanan who established the fast on the ninth day of the month did so because they felt that it was better to establish the fast day on the day of the troubles’ onset.
Based on this, they explain that the first opinion holds that there is no shavua she’chal bo in a year when Tisha B’Av falls on Shabbos and is pushed off to Sunday because the Rabbanan only argued that, when possible, the fast should be established at the onset of the troubles. However, when it is not possible to fast on the ninth day of Av (i.e., when it falls out on Shabbos), they would agree with Reb Yochanan’s view that the fast should take place when the majority of the Beis HaMikdash burnt – namely on the tenth day of Av. Based on this, the week that precedes Tisha B’Av is not the week when the fast falls out, since in a year like this year we fast on the tenth day of the month (Sunday) – which is the beginning of the following week.
The other opinion holds that the halachos of shavua she’chal bo do apply to the week prior to Tisha B’Av, even when it falls on Shabbos, because they opine that the Rabbanan hold that the fast should always be on the ninth day – even when one cannot fast on that day. The reason why we fast on Sunday is merely to make up for not being able to fast on Shabbos. However, the fast day is primarily on the ninth day. Hence all the halachos of shavua she’chal bo apply, since Tisha B’Av falls out during that week – namely on Shabbos.
This permits us to explain another machlokes, the one between the Mechaber and the Ramah (554:19) regarding whether one must keep aveilus betzina (hidden aveilus, i.e. marital relations) on Tisha B’Av that falls on Shabbos. The Mechaber says that one may have marital relations on the ninth day of Av when it falls out on Shabbos. The reason: The fast was primarily established to be on the tenth day, and the ninth day is not a fast day at all. Therefore, the Mechaber holds that one need not keep any aveilus betzina on the ninth day. But the Ramah argues that this is forbidden and that one must keep aveilus betzina since the Rabbanan established Tisha B’Av to always be on the ninth day of Av – even when one cannot fast.
There is one problem, however, with this suggestion. Why does the Mechaber say that, when Tisha B’Av falls on Shabbos and is pushed off to Sunday, there is a leniency regarding one making a bris milah? In siman 559:9, the Mechaber writes that one who makes a bris milah on a Sunday Tisha B’Av (that really fell on Shabbos) does not have to fast and may wash his body. In contrast, one must fast and may not wash his body if making a bris milah on the regularly scheduled day of Tisha B’Av. If we explain that the Mechaber is of the opinion that when Tisha B’Av falls out on Shabbos and is pushed off to Sunday (making Sunday the actual day of the fast, and thus Tisha B’Av didn’t fall in the prior week), permitting one to have marital relations on Shabbos, why is there any leniency or discrepancy regarding the fast on Sunday?