The Prophet sallAllaahu 'Alayhi wa sallam said: “by fasting the day of 'Ashuraa’, I hope that Allaah will accept it as expiation for the year that went before.”
09 Muharram = Sunday, 4th December 2011
10 Muharram (day of 'Ashuraa) = Monday, 5th December 2011
wAllahu'alam..
[Expiation = the act of atoning for sin or wrongdoing (especially appeasing a deity)]