When the 9th of Ab falls on Shabbat, the fast of Tisha B’Ab is delayed until Sunday. We eat our normal meals on Shabbat, and the Gemara establishes that even the final meal eaten before sundown may be large and festive, "like the meal of King Shelomo in his time."
An interesting question arises in the case of a boy who turns thirteen on the 10th of Ab which falls on Sunday. Is he required to fast that day? On the one hand, he is a Halachic adult on the day when the fast is required – the 10th of Ab – and should thus seemingly be required to fast. On the other hand, the fast was to have been observed the previous day, on Shabbat, and it is only because we cannot fast on Shabbat that the observance is delayed until Sunday. Perhaps, then, only those who were theoretically obligated to fast on Shabbat are obligated to fast on Sunday.
The answer to this question depends on how we perceive the observance of the fast on the 10th of Ab when the 9th falls on Shabbat. Do we view the fast on the 10th as a "makeup" for the fast which could not be observed on the 9th? Or, do we say that in such a case, the Tisha B’Ab obligation from the outset applies on the 10th of Ab, and not on the 9th. According to this perspective, we fast on the 10th not to make up the fast which we missed, but rather because in this case the Tisha B’Ab observance is scheduled for the 10th of Ab, and not the 9th.
Some Halachic authorities suggest drawing proof from the fact that some elements of mourning are observed on Shabbat, the 9th of Ab, in such a case. The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) cites a view that although one may indulge in a large meal before the fast on Shabbat, this should not be done in a joyous, festive manner, but should rather be accompanied by a somber, solemn aura. Others maintain that if one normally conducts a joyous, festive meal with friends late Shabbat afternoon, then he should do so this Shabbat, too, as this would otherwise constitute a public display of mourning on Shabbat. This discussion presumes that some degree of mourning should be observed on this Shabbat – albeit perhaps only in private – which might suggest that even in such a case, the real day of Tisha B’Ab is on Shabbat, even if the actual fast is delayed until Sunday. Accordingly, only those who would have been obligated to fast on Shabbat are obligated to make up the missed fast on Sunday.
Others, however, disagree. Rav Shemuel Wosner (1913-2015), in Shebet Ha’levi (4:72), takes the position that from the outset, the 10th of Ab is considered the date of the Tisha B’Ab observance when the 9th falls on Shabbat, and therefore, one who becomes a Bar Misva on the 10th of Ab in such a case is obligated to fast.
Summary: If Tisha B’Ab falls on Shabbat and is thus observed on Sunday, even a Bar Misva boy who turns thirteen on Sunday, and would not have been obligated to fast if Tisha B’Ab had been observed the previous day, is obligated to fast.
