Fast Day Postponed, Indefinitely.: When the 17th of Tammuz and Tisha B’Av coincide with Shabbat
The sages of the Talmud debate whether a fast day that falls on Shabbat is postponed to Sunday or dismissed entirely. In exploring these views, this class addresses the inner dimension of the commands to delight in the Shabbat and to fast on specific days. (Likkutei Sichot, vol. 33)
| Related Text: | Likutei Sichot - חלק לג: י"ז תמוז |
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In the case of the bris, the father does eat even when bris is on a fast day that was not postponed not just fast that was deferred to Sunday. The difference is that on a fast that was not postponed the father has to make up the fast he interrupted, that's the stringency, whereas when the bris occurs on a postponed fast day, the father is not required to fast again.
See sicha inside.