The more known: Source: Wikipedia.
Muharram (Arabic: المحرّم) is the first month of the Islamic calendar. It is one of the four sacred months of the year in which fighting is prohibited. Since the Islamic calendar is a lunar calendar, Muharram moves from year to year when compared with the Gregorian calendar.
Muharram is so called because it is unlawful to fight during this month, the word is derived from the word haraam, meaning “forbidden”. It is held to be the most sacred of all the months, excluding Ramadan. Some Muslims fast during these days. The tenth day of Muharram is called Yawmu-l ‘Ashurah, which is known by Shia Muslims as ‘the day of grief’.
Many Sunni Muslims fast during this day, because Musa (Moses) and his people obtained a victory over the Egyptian Pharaoh on the 10th day of Muharram [Citation needed]; according to them Islamic prophet Muhammad asked Muslims to fast on this day, and also a day extra either before or after, so that they are not similar to Jews (since, according to him, Jews used to fast for one day due to the same reason)
Fasting differs among the Muslim groupings; mainstream Shia Muslims stop eating and drinking during sunlight hours and do not eat until late afternoon. Sunni Muslims also fast during Muharram for the first ten days of Muharram, just the tenth day or on both the ninth and tenth days; the exact term depending on the individual. Shia Muslims do so to replicate the sufferings of Hussein ibn Ali on the Day of Ashura. Shia Muslims, go further in their attempts of replication, including self-flagellation (also see Matam).
The less known
Source: as the editor was told while in Baghdad
For Shias this is also a period when families meet each other more. Many household slaughter animals and prepare meals and feed the poor and the neighbors. Consequently this is also a time when mothers look out for a bride for their growing sons or probably mothers of ‘young ladies’ take them along to show the extended family that their daughters have grown up.
And such is life: joy after sorrow …