<P> Similarly, the Islamic calendar and Muslim communities recognize the convention of the IDL . In particular, the day for holding the Jumu'ah prayer appears to be local Friday everywhere in the world . </P> <P> The IDL is not a factor in the start and end of Islamic lunar months . These depend solely on sighting the new crescent moon . As an example, the fasts of the month of Ramadan begin the morning after the crescent is sighted . That this day may vary in different parts of the world is well known in Islam . (See Ramadan § Beginning .) </P> <P> The concept of an international date line in Jewish law is first mentioned by 12th - century decisors . But it was not until the introduction of improved transportation and communications systems in the 20th century that the question of an international date line truly became a question of practical Jewish law . </P> <P> As a practical matter, the conventional International Date Line--or another line in the Pacific Ocean close to it--serves as a de facto date line for purposes of Jewish law, at least in existing Jewish communities . For example, residents of the Jewish communities of Japan, New Zealand, Hawaii, and French Polynesia all observe Shabbat on local Saturday . However, there is not unanimity as to how Jewish law reaches that conclusion . For this reason, some authorities rule that certain aspects of Sabbath observance are required on Sunday (in Japan and New Zealand) or Friday (in Hawaii and French Polynesia) in addition to Saturday . Additionally, there are differences of opinion as to which day or days individual Jews traveling in the Pacific region away from established Jewish communities should observe Shabbat . </P>

Where is the international date line why is it called so