<P> In the Persian calendar, Sunday is the second day of the week . However, it is called "number one" as counting starts from zero; the first day - Saturday - is denoted as 00 . </P> <P> In Roman culture, Sunday was the day of the Sun god . In paganism, the sun was a source of life, giving warmth and illumination to mankind . It was the center of a popular cult among Romans, who would stand at dawn to catch the first rays of sunshine as they prayed . </P> <P> The opportunity to spot in the nature - worship of their heathen neighbors a symbolism valid to their own faith was not lost on the Christians . One of the Church fathers, St. Jerome, would declare: "If pagans call (the Lord's Day) (...) the' day of the sun,' we willingly agree, for today the light of the world is raised, today is revealed the sun of justice with healing in his rays ." </P> <P> A similar consideration may have influenced the choice of the Christmas date on the day of the winter solstice, whose celebration was part of the Roman cult of the sun . In the same vein, Christian churches have been built and are still being built (as far as possible) with an orientation so that the congregation faced toward the sunrise in the East . Much later, St. Francis would sing in his famous canticle: "Be praised, my Lord, through all your creatures, especially through my lord Brother Sun, who brings the day; and you give light through him . And he is beautiful and radiant in all his splendor! Of you, Most High, he bears the likeness ." </P>

Which is the first day of the week