An article in Urdu shows that the month of Muharram is one of the four sacred months as in the Quran. The caliph Omar (may Allah be pleased with him) made it the first month of the Hijri calendar. It also has the day of Ashura in which Allah saved Moses and his people from Pharaoh, so he fasted that day to express gratitude to Allah Almighty. As a result, the Prophet Muhammad (peace and blessings of Allah be upon him) fasted on that day, encouraged his companions to do so, and said, "Fasting on the day of Ashura expiates the sins of the preceding year." This does not have any relation to the killing of Hussein (may Allah be pleased with him) so it is not a month of mourning.