This year Tisha B’Av falls on Shabbos. The fast and all of the other restrictions on the day are observed on Sunday, since one may not fast or mourn on Shabbos. The general practice for most fast days is to push them off to the following Sunday. However, there are some who believe that concerning Tisha B’Av the procedure is somewhat different, as we will see.
In addition to the restrictions of Tisha B’Av, there are several restrictions that one may not perform during the week that Tisha B’Av falls in. This is referred to as shavua she’chal bo. For example, one may not take a haircut or wash his clothing and there is a custom not to eat meat or drink wine (the custom of Ashkenazi Jews is to refrain from these actions from Rosh Chodesh Av). 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 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 this 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 set on fire close to the end of the ninth day of Av and continued burning throughout the tenth day of Av. Rav 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 that the peraniyos (misfortunes) began.
Based on this, they explain that the opinion that holds that there is no shavua she’chal bo in a year when Tisha B’Av falls on Shabbos and is thus pushed off to Sunday, believes that the Rabbanan only argued that when possible the fast should be established at the onset of the troubles, namely, the ninth of Av. 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 Rav 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 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 instituted 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 explanation can shed light on another machlokes as well. The Mechaber and the Rema (554:19) disagree as to 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. Based on the above explanation we can suggest that this is because the fast was primarily established to be on the tenth day in such a scenario, 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 Rema 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.