Why Cheshvan & Kislev?

Why Cheshvan & Kislev?

The Tur (O.Ch. 428) writes ,”Tishrei is always a full month (30days)

and Teves is always a missing (short) month (29days)

The months alternate one month is full and the following is short.

The months of Cheshvan & Kislev are exceptions. Some years both 

months are full.(שנה שלמה ). Some years both months are short

( חסרה ) and some years one is full and the other is short (כסדרה)“

The reason for this is to equalize the months. A lunar cycle is

29 days+12hours+ 793 Chalokim (Aprox. ¾ hr.)

The Levush explains why did Chazal pick Cheshvan & Kislev

and not any other month when we do not always alternate full 

and short months .

He explains, 

“The first opportunity we have to add or subtract from the month

is on the month of Cheshvan.

We can’t do it on Nissan & Iyar  because the 50^th day of Sefirah

has to fall on the 6^th of Sivan.

(by adding or subtracting a day  we will have it on the 5^th or the 7^th )

From Shavuos till Yom Kippur we need to have 3 x 40 days.

to commemorate the 3x 40 days from Moshe Rabeinu standing

on the mountain (Shavuos) and bringing down the Luchos on

Yom Kippur.

Chodesh Tishrei is the king of the Chodoshim and the Month

with Yomim Tovim (ירח האתנים )  we can’t make it a חסר .

Therefore the first opportune time is in the month of Cheshvan.