The name is glossed as to revere or to hold in awe. 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.
Rajab is the sacred month that stands alone, separated from the other three, and the pre-Islamic sources call it Rajab of Mudar after the tribal group that observed it most strictly. The night journey of the Prophet is placed in this month in much of the tradition, and the association is widely observed although the dating is not agreed.