Should one do some Dinim of Aveilus (“miktzas aveilus”) for a grandparent?
Halachically there is NO Aveilus on a grandparent.
The Shulchan Aruch (Y”D 374, 6) writes that there is a custom of miktzas aveilus (some mourning) after other relatives such as a grandparent, but that’s not the custom today.
The Minhag (custom) today: in front of the Aveilim (mourners), out of respect – we act also miktzas aveilus (‘some’ of mourning behavior, i.e. refraining from joy, new clothing, etc.) up until the end of the first Shabbos.
Sources:
שו”ע יו”ד סי’ שע”ד ס”ו. רמ”א שם. ש”ך שם סק”ז. ערוך השלחן שם סעי’ טז.
וראה בפתחי תשובה שם סק”ד (ומ”ש משו”ת אדני פז סי’ י”א). קיצור שו”ע סי’ ר”ג סעי’ ב.
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