Why Sunday is considered holiday in some countries and Friday in middle east ? The observance of SUNDAY is a specifically CHRISTIAN practice, based on the practice of Christians as early as the FIRST century. The day was held as special as the day of the week on which Jesus was believed to have risen from the dead (also the day of week of the first day of the feast of "Pentecost" that year). We even seem to have a reference to this day . "the Lord's Day" in Revelation 1. Note that this as NOT something begun by Constantine (and why would the Christians simply adopt what he said anyway?) though he did make it a formal, legal day of rest. Observance of Friday, on the other hand, was the Muslim practice . note the following: "there exists no authentic and complete account of the establishment of this most important institution of Islam in the ancient sources. The only passage of the Qur'an, which refers to it ...supposes it to be already in existence. . . . "Nevertheless, a number of facts about the origin of the Friday service emerge clearly from those accounts: (1) There was no Friday service in Mecca, the 'caravan city,' in which Muhammad began his prophetic career. Al-Tabari, in his Annals, part i, p. 1256, I. 20, says so expressly, while all the other sources confirm this fact by implication. (2) Public worship was held by the new Muslims, at their own initiative, in Medina even before Muhammad arrived there in 622 and made it his permanent domicile, but it was Muhammad who ordered that it should be held regularly on Friday." So WHY was FRIDAY observed in Medina? Apparently it is connected with the 'market day' in preparation for the Jewish Sabbath (observed on SATURDAY): "The day was chosen for the simple reason that on it 'the Jews bought their provisions for their Sabbath,' i.e., it was the weekly market-day of the oasis of Medina; everybody was present, and it was, thus, a natural occasion for bringing people together for the purpose of prayer and admonition. "That Friday was the weekly Jewish market-day everywhere, except in big cities, is known from Talmudic sources.It is indeed natural that people should do their marketing on the eve of the weekly holiday.