The Rama (O.C. 686) paskens that if a milah occurs on Ta’anis Esther which is mukdam on Thursday instead of Shabbos, the seudah for the milah is held on Thurs. and the fast is postponed (for those involved) until Friday. Shouldn’t it follow that if a siyum held on ta’anis bechorim which is mukdam on Thurs., the fast should be postponed until Friday. Yet, we know nobody does that and we assume the siyum cancels the ta’anis. Why the difference?
There is a nice lomdus here, but I’ll let the question simmer so the answer is absorbed b’teyavon, like Rashi’s shita in the hidur of achilas matzah : )