1,290. If the Equinox is More Than Three Days Off

Hilchos Kiddush HaChodesh 9:7

Based on the above, let’s say that one wants to know the date of the vernal equinox in the year 4930. This is the ninth year in the 260th 19-year cycle and we have already calculated that Rosh Chodesh Nisan is a Thursday and that the equinox will occur on a Thursday. Since this is the ninth year in the cycle, that means that eight years have been completed. Adding 11 days for each year, this equals 88. We add seven for a total of 95. Divided by 30, this yields a remainder of five. We add five days to Rosh Chodesh Nisan – a Thursday – which lands us on a Monday. Since we know that the equinox will fall not on a Monday but on a Thursday, we add days until we reach Thursday, which is the day of the equinox. We therefore determine that the vernal equinox of this year will occur on 8 Nisan. This procedure can be applied for any year.

Hilchos Kiddush HaChodesh 9:8

Even though we said that one add days until he reaches the day on which the equinox is known to occur, it should never be necessary to add more than three days, in extremely rare cases four. If you need to add any more days than this, then you have made a mistake and should start again.