Siman 492

The Tur wrote: It is customary in Ashkenaz and France to fast Monday, Thursday, and Monday after Pesach and Sukkot. And they wait until the entire month of Nisan and Tishrei has passed and then they fast, because they do not wish to fast in Nisan and Tishrei. And they based them on the verse in Iyov: "And it was, when the days of feasting had gone round... perhaps my children have sinned...". And because the days of the festival are days of feasting and joy – perhaps they sinned. And they treat them like a public fast, to read "Vayechal". Thus far his words. And there is an allusion in the Gemara: The stumbling block of the year is the festival (Kiddushin 81a). And our teachers, the authors of the Tosafot, explained, and this is their wording: The shaky time of the days of the year regarding seclusion and transgression is all the days of the festival... therefore it is customary to fast after Pesach and after Sukkot. Thus far their words. And after Shavuot they were not concerned, since it is only two days even in the diaspora, and there is not so much time for sin.

And the custom is that on the first Shabbat after Rosh Chodesh Iyar or Marheshvan – they announce the fasts. And they say "Mi Sheberach..." as printed in the prayer books, because we have not accepted them upon ourselves as an obligation. And for this reason, now, due to the weakness of the generations, those who fast have decreased. And one who answered "Amen" does not need to accept the fast on Sunday and on Wednesday at Mincha, for this is like an acceptance. But the "Amen" does not compel him to fast as some mistakenly say, for the "Amen" is nothing but a confirmation of a blessing for whoever volunteers to fast. Rather, this is no less than an acceptance at the time of Mincha. And in siman 566 and 568 these laws will be explained with the help of Heaven. And when there is a milah or a wedding – they announce at Mincha (Magen Avraham).