The sabbath day is holy for Jews. Jews take break on the seventh day, because God rested on the seventh day after his creation and enjoyed it. He wants us to rest. He didn't rest because He was tired, but to set an example for us that our body need to rest.
Does it have to be on Sunday? No. For example your pastor might take the sabbath day on Friday because he has a lot of works of Sunday.
To my family, it's not Sunday, but Friday, although it's not sabbath day. It's special day for Muslims.
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