Adoption of the Gregorian calendar started to make the change from the Julian calendar and towards the Gregorian calendar in 1700, but it was decided to make the (then 11-day) adjustment gradually by excluding the leap days (29 February) from each of 11 successive leap years, 1700 to 1740. Meanwhile, the Swedish calendar would be out of step with both the Julian calendar and the Gregorian calendar for 40 years; also, the difference would not be constant but would change every four years. This system had potential for confusion when working out the dates of Swedish events in this 40-year period. To add to the
Hebrew calendar the basic Hebrew calendar year is one of twelve lunar months alternating between 29 and 30 days: In leap years (such as 5779) an additional month, Adar I (30 days) is added after Shevat, while the regular Adar is referred to as "Adar II." The insertion of the leap month mentioned above is based on the requirement that Passover—the festival celebrating the Exodus from Egypt, which took place in the spring—always occurs in the [northern hemisphere's] spring season. Since the adoption of a fixed calendar, intercalations in the Hebrew calendar have been assigned to fixed points in a 19-year cycle.