The name is glossed as forbidden or sacred. It is one of the four sacred months. The twelve month names are Arabic and predate Islam, retained from the calendar of the Arabian peninsula when the Hijri reckoning was established.
Four of the twelve are the sacred months, al-ashhur al-hurum, in which warfare was forbidden in pre-Islamic Arabia and which the Quran confirms at sura 9 verse 36: Dhu al-Qadah, Dhu al-Hijjah, Muharram and Rajab. Three run consecutively and the fourth stands alone in the middle of the year, an arrangement that fitted the pilgrimage season and the trading truces that went with it.
The Hijri calendar admits no intercalation. The verse immediately following the one that names the twelve months condemns nasi, the pre-Islamic practice of postponing or adding a month, and the calendar has run as a strict lunar reckoning since. The consequence is a year of 354 or 355 days that moves against the seasons by about eleven days annually, so any month passes through the whole solar year in roughly thirty-three years.
Muharram opens the year, and the tenth of the month, Ashura, is marked across the Muslim world, as a voluntary fast in Sunni practice and as the commemoration of the death of Husayn at Karbala in Shia practice. The atlas records both observances as they are held rather than describing one through the other.