Nabi (sallallahu alayhi wasallam) said: “By Allah, there is no better time than the ten days of Zil Hijjah, wherein one fast (kept in these days) is equal to a whole year of fasting and one night of worship is equal to the Night of Power (Laylatul Qadr). (Tirmizi)
One day from among the ten days is the Majestic Day of Arafah. Fasting on this day (9th of Zil Hijjah), for example, compensates for the past year and the next year's sins.
Mas'ala – It is Mustahab (recommended) for a person slaughtering an animal not to clip the nails and cut/shave the hair. This is in resemblance to the person in Ihram fulfilling the magnificent pillar of Islam, Hajj.