Celebrating death anniversaries

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Tomorrow is my grandfather's 9th death anniversary. I think my mother is going to prepare a little something for tomorrow. I just do not understand why we celebrate people's death anniversaries, however, there is nothing special done on their birthday. I think I would prefer it if when I am already gone, they'd be celebrating my birthday rather than my death anniversary. Do you celebrate death anniversaries of your loved ones? Why? I don't understand at all.