Basic Facts Lunar Calendar. The Jewish Calendar is lunar in character. The months are reckoned according to the time that it takes the moon to travel round the earth. Each month consists of either 29 or 30 days. The lunar year consists of approximately 354 days, while a solar year (the time it takes the earth to make a complete circuit round the sun) consists of 365 days. Therefore, some adjustment of the lunar year has to be made to match it to the solar year. If there would be no adjustment, the religious calendar would become variable. Thus, for example, the Festival of Pesach, which should always occur in the spring (in Israel), would occur earlier by eleven days each succeeding year until the Spring Festival would be kept in winter! Leap years. So, the adjustment is achieved by inserting an extra month in the lunar calendar seven times in nineteen years. A year of 12 months is called Shanah Peshutah (an ordinary year), while a year of 13 months is called Shanah Me'uberet (a leap year). In a cycle of 19 years, a leap year occurs on the 3rd, 6th, 8th, 11th, I4th, I7th and 19th years.