Each year on the 9th day of the Islamic month of Dhu al-Hijjah, after an afternoon of prayer at Arafat, Muslim pilgrims visit Muzdalifah in the evening. Because of over-crowding, the pilgrims sometimes do not arrive there until late at night. At Muzdalifah they collect pebbles (the number 70 is chosen because there are three pillars that represent the devil and you throw seven at the large one on day one, and seven at all three on the next two or three days) which will be thrown in the Stoning of the Devil ritual in Mina during the next three or four days. The pilgrims spend the night at Muzdalifah, often sleeping in the open air, before leaving for Mina the next morning. Muzdalifah is preceded by a day at Arafat, glorifying Allah, repeating the Supplication, repenting to Allah and asking Him for Forgiveness. In Arafat one must pray the Zuhr and Asr Prayers shortened and combined during the time of Zuhr. After sunset on the 9th day of the Islamic month of Dhu al-Hijjah, pilgrims travel to Muzdalifah.
The open-roofed mosque is known as al-Mash'ar al-Haram (the Sacred Grove).
^David E. Long The Hajj Today: A Survey of the Contemporary Pilgrimage to Mecca 1979 "With thousands of Hajjis, most of them in motor vehicles, rushing headlong for Muzdalifah, the potential is there for one of ... There is special grace for praying at the roofless mosque in Muzdalifah called al-Mash'ar al-Haram (the Sacred Grove) ..."
^Danarto A Javanese pilgrim in Mecca 1989 Page 27 "It was still dark when we arrived at Muzdalifah, four miles away. The Koran instructs us to spend the night at al-Mash'ar al-Haram. the Sacred Grove at Muzdalifah, as one of the conditions for the hajj . We scrambled out of the bus and looked ...
^Lindsay Jones Encyclopedia of religion - 2005 Volume 10 - Page 7159 "The Qur'an admonishes: "When you hurry from Arafat, remember God at the Sacred Grove (al-mash' ar al-haram)," that is, at Muzdalifah (2:198). Today a mosque marks the place in Muzdalifah where pilgrims gather to perform the special saldt ..."
^Ziauddin Sardar, M. A. Zaki Badawi - Hajj Studies 1978 -- Page 32 "Muzdalifah is an open plain sheltered by parched hills with sparse growth of thorn bushes. The pilgrims spend a night under the open sky of the roofless Mosque, the Sacred Grove, Al Mush'ar al-Haram. On the morning of the tenth, all depart ..."