A mourner may not generally enter a hall of joyous celebration and may not eat at any public meal. During the year of mourning for parents, you may not join any public festivities (even if it is not a simcha) that have a meal, including any meals celebrating a mitzva (se'udat mitzva) such as for a brit mila, wedding, or redemption of a son (pidyon ha'ben). After 30 days, you may attend a bar mitzva or a siyum meal, since a bar mitzva is similar to a siyum since the child's parent's commandment to educate his/her child in Jewish education has been completed.
ExceptionA mourner whose child is getting married, does attend the wedding and does eat at the meal with everyone else, even if mourning for a parent. He or she does not need to leave the room when music is being played. To attend the wedding of anyone other than one's child, regardless of who died, a mourner must eat alone and outside the main dining area.