If one is invited to bris, is it a halacha they have to go, where is the source? Does the same principle apply to any seudas mitzvah?
The Rema writes (Yoreh De’ah 265:12) that whoever does not participate in the festive meal that accompanies a Bris is viewed as if he is “excommunicated from Heaven.” He adds that if offensive people are participating in such a meal, one is not obligated to join them.
Because of this ruling, some are careful not to direcly invite people to a bris (“You are invited to the Bris”), but rather inform their friends and community of the bris (“The Bris will take place on such-and-such a day, at such-and-such an hour”).
This is done so that if the guest is unable to participate in the ceremony, his declining the invitation will not be interpreted as a refusal to take part in the festive meal, thereby sparing him the rist of “excommunication from Heaven” (Pischei Teshuvah, 265:18).
However, there are opinions that dispute this ruling, and argue that there is no problem of inviting people to a bris, because the statement of the Rema applies only to guests who are present at the bris — at the meal itself — and refuse to take part. By so doing, they show disdain for the mitzvah, which cannot be said for somebody who is absent from the entire affair.
Shut Sho’el Ve-Nish’al (Vol. 7, Yoreh De’ah 209) writes that this (lenient) ruling emerges from the wording of the Rema itself, who writes that somebody who does not participate in the bris is considered as though excommunicated from Heaven. The next sentence, stating that where offensive people are present one need not join the meal, implies that the reference is to somebody actually present at the meal.
Note also that the Kaf Ha-Chaim (Sofer, 90:67) writes that even when formally invited to a meal, one who needs to daven with the tzibbur (congregation) should not pass up on his obligation, and he is not looked upon as one who refuses to take part in the commandment.
There are also opinions that if there is already a quorum of ten adult Jewish males at the meal, the guest is not obligated to take part, for the commandment will at any rate be fulfilled without him (see Otzar Ha-Bris, p. 163).
For some, the lack of a formal straightforward invitation can be misinterpreted, and liable to make him feel unwanted. In such cases, it is certainly better to invite him in a clear and unmistakable manner.