Should one who fasts or recites Selichos on Behab after Sukkos and Pesach also do so after the third Yom Tov, Shavuos?
This ancient custom dates back to the era of Tosafos (Kiddushin 81a), who mentions it only on Sukkos and Pesach. Let's explore some reasons for this and see if they apply to Shavuos.
The Tur (492) mentions that since the Chagim are periods of joy and feasting, there's a concern that it may lead to transgressions, so the fast is observed as an atonement. Tosafos (ad loc.) highlights that potential sins could involve inappropriate interactions between men and women during the festivities.
Levush (492) suggests fasting can benefit one's health as the seasons transition, safeguarding against potential risks associated with seasonal changes.
The Eliya Rabba (492:3) says that since it's easy for a person to make mistakes with the Halachos of Yom Tov, such as working on Chol Hamoed or cooking on Yom Tov when not permitted, we fast to atone for those sins.
The above reasons don't apply to Shavuos since it's a short Chag. There is less concern that it will lead to sin and no worries about seasonal changes. Therefore, the commonly accepted practice is to refrain from fast Behab after Shavuos.
And that's how Tosafos (Kiddushin 81a), Tur- Shulchan Aruch (492:1), Levush (ibid), Aruch HaShulchan (ibid), all rule: there is no Behab after Shavuos.
However, Chok Yaakov (492:1), Eliya Rabba (492:4), and Kaf Hachaim (492:1) all bring the Ravad (Tomim Deim) that says, "people of stature" may fast even after Shavuos. Pri Megadim (A.A. 492:1) states that those who fast Behab after Shavuos should not fast until after the 13th or 14th of Sivan.