ROME, July 8 (JTA) — Pope John Paul II has relied on Jewish tradition to explain to Catholics why they should celebrate Sunday as a holy day. In an apostolic letter called “Dies Domini,” or “The Day of the Lord,” the pope outlined the history and importance of Sunday in Catholic religious belief and detailed its roots in the Jewish Shabbat. In the 100-page document, he traced the origins of Shabbat observance to the Book of Genesis and noted that Sabbath observance was enshrined in the Ten Commandments. “In setting this commandment within the context of the basic structure of ethics, Israel and then the Church declare that they consider it not just a matter of community religious discipline but a defining and indelible expression of our relationship with God,” he said. He also told the history of how, in the early centuries of Christianity, the Christian Sunday became distinct from the Jewish Sabbath even though “there have always been groups within Christianity which observe both the Sabbath and Sunday as ‘two brother days.’ ” In his letter, the pope urged the world’s 1 billion Catholics to “rediscover Sunday” as a day for prayer, contemplation and charity as well as for rest and forms of entertainment that offer “spiritual enrichment.” He made his call in response to what he called the “strikingly low” percentage of Catholics who celebrate Mass on Sunday in some parts of the world.