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Is Shabbat over when the sun has gone down or is Shabbat over after the Havdalah ceremony?
If it is the latter, does the Shabbat keep going for the rest of the week? Since the Torah says that Shabbat is on the last day of the week, could one have Shabbat on the wrong day? Would it still be Shabbat?
Is Havdalah a good thing or something you need to in order to end Shabbat?