Question:
Can I attend a seudah of a friend's bar mitzvah on my father's yortziet?
ראיתי בש"ך שאין לאכול בסעודת משתה ושמחה ביום היארצייט. השאלה היא אם זהו בכלל סעודת משתה ושמחה, ואת"ל שהוא בכלל, האם האיסור הוא רק לאכול וכלשונו, אבל מותר לשבת שם.
Answer:
It is permitted, because this minhag only applies to not attending a wedding, but not for other simchos or seudos.
Sources:
There are different opinions if it permitted or not. From the terminology of the Rema Y:D 391-3 that a person should not go to any seuda on the yahrtziet of a parent. The Piskei Teshuvos ( ibid-8), brings others that say that this is only for a chasuna but not for other seudas mitzvas. This is also the ruling of Kitzur Shulchan Aruch 221-7, Pri Megadim O:CH 447- 9. Being that we are dealing here with a minhag, being that it is controversial if it applies or not we can be lenient. This is the ruling of a numerous poskim.