It was reported from Abu Qutaadah (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) was asked about fasting on the Day of Arafaah. He (sallallaahu alayhi wa sallam) said,

It expiates for the sins of the previous year and of the coming year.      

(Reported by Muslim)