If you follow the numbers of the days in Hebrew you will note that ‘Alef’ is one, ‘Bet’ is two and so on until ‘Tet is nine and ‘Yod’ is ten. Following that ‘Yod' ‘Alef” is eleven, and ‘Yod’ ‘Bet’ is twelve and so on. When you get to the number fifteen in the progression the sequence would require ‘Yod’ ‘Hei’ the letters of ‘ten’ and ‘five’.
This however spells ‘YAH” an abbreviation for the sacred name of YHVH. This is too sacred to be used as merely a number and to avoid the danger of breaking the command of Exodus 20 vs. 7, the sages of old changed the numeral fifteen to read ‘Tet’ ‘Vav’ and the letters for sixteen to read ‘Tet’ ‘Zayin’.
This is because the numerical value for ‘Tet’ is nine and ‘Vav’ is six, which together equals fifteen, and likewise ‘Tet’ (9) and ‘Zayin’ which is seven equals sixteen. The sequence resumes normally again from seventeen with ‘Yod’ ‘Zayin’.
Baruch Hashem- Blessed be “The Name”
Shalom and welcome to this presentation of what is a very ancient calendar system
