Q. I was invited by a neighbour to attend a BBQ in his Back Yard on the Monday that was the first day of BEHAB (the three days after Pesach, namely Monday, Thursday and Monday, many recite Selichos). Can I attend?

A. Shulchan Aruch (O.H. 492: 1) mention that there is a tradition for some to fast three days after the end of Pesach or Sukos, to atone for any misgivings done during the great rejoicing of Yom Tov.

Rema (ibid) maintains that it is the actual tradition of congregations of Germany and France. So also maintains Shulchan Aruch Horav.

Eliahu Rabah, and others quoted by Nitei Gavriel (43: 13) opine that even those who do not fast, should avoid eating meat and drinking wine. Therefore, it is improper to make a BBQ during those three days.
Horav Shlomo Miller’s Shlit’a opinion is similar.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller, Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a