Why Taasuaa (تاسوعا) is important for Shia?

There are two days that are so significant for Shiite as **"Taasuaa/تاسوعا And عاشورا/Ashura"** ... Actually usually they use and name these two days successive. (I am aware that Imam Hussein martyred at the tenth day of Moharam as Ashura), but I wonder what the importance of Taasuaa or تاسوعا is? _______________________________________________________ **References:** www.ahmadinejad.ir www.pasokhgoo.ir