Catholic Answer
It was known as the Cluniac reform and it began with the great Abbey of Cluny in France which was founded in 910 A.D. Historically, most monasteries in the Catholic Church are Benedictine, and at one time Benedictine Houses dotted Europe. See the link below for the story of Cluny and pictures of what remains today.
from Modern Catholic Dictionary by John A. Hardon, S.J. Doubleday & Co., Inc. Garden City, NY 1980
Cluny, Order of. A branch of Benedictines founded in 927 by St. Odo (879-942), who organized the reform of monasticism in France. The object of the Cluniac reform was a return to the strict rule of St. Benedict, the pursuit of personal sanctity, chanting the Divine Office in choir, solemnity in divine worship, and corresponding reduction of manual labor. The historic reforms of Pope Gregory VII (Hildebrand 1073-1085), who had stayed in Cluny for a time, affected the life and discipline of the whole Catholic Church. By the later Middle Ages, the influence of the Cluniac spirit waned mainly because of interference by political powers and the confiscation by the state of monastic holdings.
The answer will depend on which country the question is about.
The first millennium began in 1 AD and ended 1000 AD. The second millennium began 1000 AD and ended 2000 AD. The third millennium (The current millennium) began 2000 AD and will end in 3000 AD.
intruduced reform
it began in 1200 AD and ended in 1532 :-) :-) .
Martin Luther
Islam began in the Arabian city of Mecca in 610 AD
Construction began on the Colosseum in 72 AD. It was finished and dedicated in 80 AD.
around 300 ad
when someone made it up
Christianity
The medieval period began in 476 AD.
Pope Innocent III in 1198 AD.