Ibn 'Abbas (may Allah be pleased with them both) said: "I never saw the Messenger of Allah (peace and blessings of Allah be upon him) so keen to fast any day and give it priority over any other than this day, the day of 'Ashura', and this month, meaning Ramadhan." [Reported by al-Bukhari, 1867]
The meaning of his being keen was that he intended to fast on that day in the hope of earning the reward for doing so. The Prophet (peace and blessings of Allah be upon him) said: "For fasting the day of 'Ashura', I hope that Allah will accept it as expiation for the year that went before." [Reported by Muslim, 1976] This is from the bounty of Allah towards us: for fasting one day He gives us expiation for the sins of a whole year. And Allah is the Owner of Great Bounty.