For what purpose is a leap year made

Hello, In Parashat Emor, 23:2, Rashi explains that the leap year was made so that the Passover holiday could be postponed due to the needs of exiles who were uprooted from their places to ascend to Jerusalem and had not yet arrived. A. If this is the case, could it happen every year, not just once every 4 years as it is today, and is it subject to the discretion of the Sanhedrin in that year? B. It is known that today it occurs once every few years so that the Passover holiday falls in the spring. How do both reasons coexist then?

Hello 

Rashi did not provide all the reasons for making or not making a leap year, but only the part required from this verse, that a leap year can be made so that all those coming from other countries can arrive. 

Of course, the main reason for the leap year is to ensure that Passover falls in the 'spring', which would not happen without the additional months. 

Since the days of the Amoraim, when Hillel the Amora established the determination of months and leap years, the leap year is not determined by changing circumstances but is fixed in advance to maintain the month of spring. The sages calculated that for this purpose, a 19-year cycle is needed, in which the years 3, 6, 8, 11, 14, 17, and 19 are leap years.  

See Tractate Sanhedrin 11-12. Rambam, Kiddush HaChodesh, Chapter 4. 

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