Prophet Mohammed (peace and blessings be upon him) has said about the first ten days of Zul Hijjah: There are no days in which righteous deeds are more beloved to Allah than these ten days." The people asked, "Not even Jihad for the sake of Allah?" He said, "Not even Jihad for the sake of Allah, except in the case of a man who went out to fight giving himself and his wealth up for the cause, and came back with nothing (Bukhari).
So sacred are the first ten days of Zul Hijjah that Allah swears an oath by them when He says in the Quran: "By the dawn; by the ten nights" [al-Fajr 89:1-2]. Swearing an oath by something indicates its importance and great benefit.
OK....so when you look at the Calendar above...you will see the first ten days in what we know as February...the FIRST day of the 10 days is marked with the ZH.