There is a common practice amongst Muslims to fast on the 10th of Muharram (1 month of the Islamic calendar). Although there are several hadith in support of this act, they can be summarized using the 2 primary hadiths below:
“When Allah's Apostle arrived at Medina, he found the Jews observing the fast on the day of 'ashura' (10th of Muharram). The Prophet asked them (about it) and they replied, "This is the day when Moses became victorious over Pharaoh." The Prophet said (to the Muslims), "We are nearer to Moses than they, so fast on this day."
Volume 3, Book 31, Number 222: Narrated by Ibn 'Abbas
Volume 4, Book 55, Number 609: Narrated by Ibn 'Abbas
Volume 5, Book 58, Number 279: Narrated by Ibn 'Abbas
Volume 6, Book 60, Number 202: Narrated by Ibn 'Abbas
Volume 6, Book 60, Number 261: Narrated by Ibn 'Abbas
Sahih Bukhari
“During the Pre-lslamic Period of ignorance the Quraish used to observe fasting on the day of 'ashura', and the Prophet himself used to observe fasting on it too. But when he came to Medina, he fasted on that day and ordered the Muslims to fast on it. When (the order of compulsory fasting in ) Ramadan was revealed, fasting in Ramadan became an obligation, and fasting on 'ashura' was given up, and who ever wished to fast (on it) did so, and whoever did not wish to fast on it, did not fast.”
Volume 6, Book 60, Number 31: Narrated by Aisha
Volume 3, Book 31, Number 117: Narrated by 'Aisha (similar to above hadith)
Volume 3, Book 31, Number 220: Narrated by Aisha (similar to above hadith)
Volume 5, Book 58, Number 172: Narrated by 'Aisha (similar to above hadith)
Sahih Bukhari
Let’s investigate the first hadith – fasting in observance Moses victory.
In Judaism, this event is known as Passover. The eight-day festival of Passover is celebrated in from the 15th through the 22nd of the Hebrew month of Nissan. The Jews of Medina also followed this calendar and not the Arab calendar. Authentic hadith tell us that the Prophet (saw) migrated from Mecca to Medina at the end of Safar so his first Ashura in Medina was in AH2. Based on on-line date conversion software (+- 1-2 days):
10 Muharram AH2 (Islamic date) = 14 July 623AD (Gregorian date) = 8th of Av, 4383 (Jewish date)
Since the month of AV is the fifth month of the Hebrew calendar, it is highly unlikely the Jews were fasting in observance of Passover on the 10th of Muharram since the dates were nowhere close to each other.
Now for the 2nd hadith – continuing the custom of Qurayish by fasting on 10th Muharram
The Prophet (saw) spent 23 years of his preaching Islam and abrogating the customs of the kuffar of Mecca yet he continued this custom even after hijra? Highly unlikely!
Even if the above traditions are taken to be true, why only celebrate one Jewish custom out of thousands of them? Also, if the Prophet (saw) celebrated their joyous occasions, did he observe and show sorrow during their sad or calamitous days? Why not observe the Sabbath or celebrate the birthday of UZA?
The entire Muslim community has a choice: either celebrate Ashura by fasting and celebrating in allegiance to the Jews of Medina and Kuffar of Mecca or observe the calamity that befell the progeny of the Prophet (saw) in a solemn and somber manner. BTW, one of the options makes you a munafiq (hint: loving the Prophet but ignoring the calamity of his progeny)