My cousin (by marriage) is having a celebration / graduation party for her daughter during the nine days – this Sunday. I’ve apologized for not being able to attend, but they don’t believe me that I am holding by the Halacha not to attend a party during this time. Can you please provide the Halacha so I can show them?
You are correct, and the graduation / celebration should really not be done during the nine days, and the reason is because we don’t make parties etc. (called simchas or seudas mirayus) during this period. (See Sefer Haorah (Rashi) 64 and Meiri Taanis 27b, brought in Kol Hamisablim Boh chapter 9 ftnt. 3.) Nevertheless, although it shouldn’t be done now, (unless it can’t be done at another time, or it will be considered a seudas mitzvah) if your cousin will be insulted by your not coming, then your attending would be considered a davar haaved, and you are permitted to attend.
Rav Y. Cahen shlit”a.