The berit mila is one of the most important and joyous events in the life of the parents and family of the newborn baby.
The ceremony is always followed by a se'uda: a festive meal or banquet. This banquet is considered a se'udat mitzva, i.e. by taking part of this meal, we fulfill a Mitzva.
The Rabbis indicated, therefore, that the participation in this meal is not optional but virtually mandatory. Rama (rabbi Moshe Isserles), in his notes to the Shulchan Arukh asserted that one who does not participate of this important banquet 'should be banned from heaven'. That is why in the Ashkenazi communities, the hosts are very careful not to 'invite' formally any guest to the Berit Mila, because if they are invited and do not come to the seuda it could be considered as if they rejected this Mitzva, and would be under a heavenly ban. In the Ashkenazi communities, then, the tradition is that people are not formally invited to the Berit Mila. The hosts just announce publicly the date and place where the berit mila will take place (pitche teshuba).
Because this banquet is not a regular meal, but the fulfillment of a Mitzva, people who are in mourning, after the shiba (the first seven days of mourning) are allowed to participate of the ceremony and the meal (when no music is played at the banquet). According to rabbi Refael Pinchasi (chayim vechesed), before thirty days (sheloshim) the mourner participates of the ceremony, but he should not participate of the banquet.
There are many different Minhagim (customs) on this issue. Rama, for example, says that during the twelve months, the mourner should not take part of any meal outside his house (YD 393), even if is a se'udat mitzva. Each one should know and follow his or her community's traditions.