A reader asks, “If this Shabbat is Hod veHadar Instrumental Kabbalat Shabbat, and I am in mourning, am I allowed to go?”
The answer basically is that we do not mourn officially during Shabbat, we observe Shabbat. To a large extent, mourning is suspended.
There is a tradition of mourners not entering the Shabbat (Friday) evening service until after the beginning Psalms which welcome Shabbat. Then when the mourners enter, the congregation offers consolation as they welcome them in, continuing with the Shabbat service.
Although Shabbat counts as one of the official seven days of shiv’ah, some practices of mourning are not observed, while some continue apace.
Mourners who do not shower or otherwise attend to their own physical appearance during the period of mourning would not be expected to do so prior to attending Shabbat services. Mourners do not attend any form of entertainment, including concerts, plays, dances, etc., while in mourning, though of course we also would not do so on Shabbat anyway. They may attend rites of passage, especially if arranged prior to the recent death, but they should not participate in the festive meals and parties associated with the occasions. There are those who would say, “Go, but don’t dance.”
Let me note with some qualification, however, that some seem to put forth that we should accommodate to a large extent what a mourner feels is right in any extreme situation such as a wedding.
Regardless, the point of a period of mourning is for the mourner to really mourn, to be free from the constraints of regular life and be tended by others. The rules are meant to help with that - for instance, covering mirrors so one doesn’t preen, and not greeting the mourner but speaking with the mourner only when the mourner initiates.
Shabbat, however, including musical instruments playing at shul, is different. We are talking about a part of the Shabbat service, not a concert. The musical instruments are in service of hiddur mitzvah, the beautification of the mitzvah; their use heightens the mitzvah. As Psalm 150 suggests, we should praise the Almighty with horns, harp, lyre, drum, flute, and lute; with dancing and with cymbals banging and clashing. We may do so even when in mourning.