LifeCycle Events: Death, Burial, and Bereavement: Burying a Person By Sundown:
What is the reason behind the law that Jews must be buried by sundown the next day?
There is no actual law that Jews must be buried by sundown the next day, although the custom is to bury the deceased as soon as possible. In Jerusalem the custom IS not to leave a dead person overnight because of the holiness of the city, and the custom has been to bury the deceased usually within 24 hours of their death. In other places it's quite common to postpone funerals so that relatives from other places may attend, although it is not preferable.
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