As a reminder for Sunni Muslims, the days for fasting in observance for Ashura will be on December 26th and December 27th, 2009.

The prophet observed the fast on Ashuraa (the 10th of Muharram),
and ordered (Muslims) to fast on that day. (Agreed upon Hadith i.e. Bukhari & Muslim).Narrated by Abi Katada: The prophet was asked about fasting on ‘Ashuraa’ (the 10th of Muharram), he said: “it expiates the previous year (for sins).” (Sahih Muslim)

It was proven from the Prophet (peace and blessings of Allaah be upon him) that the best fasting after Ramadaan is fasting in the month of Muharram. It was narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allah be upon him) said: “The best fasting after Ramadaan is the month of Allaah Muharram, and the best prayer after the obligatory prayer is prayer at night.” Narrated by Muslim, 1163.

Narrated by al-Bukhaari (1865) from Ibn ‘Abbaas, who said: The Prophet (saws) came to Madinah and saw the Jews fasting on the day of Ashoora. He said, “What is this?” They said, “This is a good day, this is the day when Allah saved the Children of Israel from their enemy and Moosa fasted on this day.” He (the Prophet Muhammad) said, “We are closer to Moosa than you.”

So he (the Prophet Muhammad) fasted on this day and told the people to fast.

There are many other versions of this Hadith in the books of “Muslim” and “Bukhari”.

According to a version narrated by Muslim,

This is a great day when Allah saved Moosa (Moses) and his people and drowned Pharaoh and his people.”