How come 어젯밤? not 어제밤. Special letter 'S' in korean. (사이시옷)

Q. How come 어젯밤, not 어제밤?

A.

We have a special 'S(ㅅ, si-ot)' in Korean orthography. 

It is called 'sa-i-si-ot'#(사이시옷) which means the 'S(ㅅ)' between words. 

We add the special letter 'ㅅ' when a changing pronunciation phenomenon occurs in two words. 

There are three cases of sound change. 

First, when the forward ends in a vowel, the first sound behind becomes double sound (ㄲ, ㄸ, ㅃ, ㅆ, ㅉ, fortis).

For example, 

비 (bi, means rain) + 소리 (so-ri, sound) 

= [비쏘리] (bi-sso-ri) 

and write it "#빗소리 (the sound of rain)". 

Second, the sound [ㄴ] is added in forward letter when it is in front of the sound [ㄴ] or [ㅁ] of the next letter. 

코 (ko, nose) + 물 (mul, water) = [콘물, kon-mul]. We write it "#콧물(runny nose)". 

Third, the double sound [ㄴㄴ] is added when it is in front of the 'Y (ㅣ)' vowels of the letter behind. 

나무 (na-mu, tree) + 잎 ( yip, leaf) = [나문닙, na-mun-nyib] and write it "#나뭇잎". 

These three sounds changing cases, we put the 'ㅅ' letter in between. 

The first letter should end with a vowel and one of the two words must be original Korean words, It can not be used when both words from Chinese letters. 

There are only six exceptions, though.
숫자 (數字, number), 횟수 (回數, number of times), 셋방 (貰房rented room), 찻간 (車間, compartment, train car) 곳간 (庫間, storeroom), 툇간 (退間, hatch? partition?)

Only 숫자, 횟수 are common. Others are not common anymore.