The Prophet, sal Allahu alayhi wa sallam, was very keen to fast on this day - so we too should be keen to fast it.
Ibn Abbas, radi Allahu 'anhu, said: “I never saw Allah's Messenger, sal Allahu alayhi wa sallam, so keen to fast any day and give it priority over any other than this day, the day of ‘Aashooraa’, and this month, meaning Ramadaan.” - Bukhari
The Arabic word for 10 is 'Asharah.
Ashooraa' literally means: the TENTH, i.e. in this case the 10th of the Islamic calendar month of Muharram.
When Allah's Messenger, sal Allahu alayhi wa sallam, moved to Madinah, he found the non-Muslims there celebrating on that day. Their reasoning? Because, they said, that this was the day Allah saved Musa from Fir'own.
Allah's Messenger, sal Allahu alayhi wa sallam, responded by venerating this day through fasting, and commanded the Muslims to fast on this day as well. (But to be different from others by fasting an 'additional' day, before the 10th. But it's alright if you didn't fast Saturday Dec.26, you can still fast Ashura.)
What: Fast on Ashura, which is the 10th of the Islamic calendar month of Muharram.
Who: YOU :)
When: Sunday, December 27, 2009 (From Fajr till Maghrib)
Why: Because the Prophet, sal Allahu alayhi wa sallam, did it, was very keen to do it, and encouraged his Ummah to do it. And because it wipes away the previous years sins.
With best wishes to see you succeed at the highest level!
- Muhammad Alshareef