The simplest prototypes of the World Double Calendar - double official state calendars of Israel, the United Arab Emirates, Saudi Arabia etc.: i.e. solar + lunar (lunar-solar) calendars working officially simultaneously. Unfortunately, in the given calendars the 7-day's week based on a Hebrew myth, not reflecting a natural lunar cycle is applied, that practically detains realization of the World Calendar reform. The World Dual PETIN-METON Calendar is organized in such a manner that the same date of any lunar month always falls on the same day of a week and always coincides with the same Moon phase. Date 1 of each lunar month always falls on Monday and always coincides with a New Moon Day. Memorials, birthdays, religious and national holidays, seasonal works, historical chronology etc. are taken into account in the World Dual PETIN-METON Calendar in accordance with a built-in solar Gregorian calendar.