As we learned in the first Mishna in Massekhet Sanhedrin, different courts were established to deal with different types of cases, with courts of three, 23 or 71 depending on the case. One situation where we find a disagreement relates to ibur shana – establishing a leap year in the Jewish calendar. When discussing ibur shana, Rabbi Meir rules that a court of three judges suffices; according to Rabban Shimon ben Gamliel they begin deliberations with three, debate the matter with five, and conclude with seven judges. Nevertheless, if the decision was made with three judges it will also suffice.
While the commonly used calendar – the Gregorian calendar – is a solar calendar that reflects the relationship between the earth and the sun (a single revolution of the earth around the sun – just over 365 days – is considered a year), and the Muslim calendar reflects the relationship between the earth and the moon (a single revolution of the moon around the earth – about 29 days – is considered a month), the Jewish calendar combines the two by establishing lunar months that must coincide with the solar year.
A leap year in the Jewish calendar involves the addition of an extra month, which serves to keep the lunar calendar in sync with the solar calendar. The court must decide – based on whether the Passover holiday will occur in the Spring, as required by the Torah (see Devarim 16:1), together with other factors – when to add the extra month.
It should be noted that in our present situation, with neither an operating Sanhedrin nor the ability of a single group of Sages to represent the Jewish People, we rely on a set calendar established by Hillel the Second, which is based on astronomical calculations. According to the set calendar, the extra month is inserted seven times every 19 years.