Jews and Muslims and Christians regard particular days of the week as holy. For Muslims this is Friday, Jews (and a few Christian denominations) regard Saturday as the Sabbath, while most Christians observe Sunday as "the Lord's Day" in honour of the resurrection of Jesus on that day. This works well if the traveller stays on the same side of the date line, but there is a problem when travellers move across it. For the traveller it might still be Friday, but the place where he is might say that it is Saturday.