Why is the ninth month called September?
"improvement, alteration for the better," late 14c., "restoration;" mid-15c., "improvement," from Old French reformacion and directly from Latin reformationem (nominative reformatio), noun of action from past participle stem of reformare (see reform (v.)). In reference to the European religious movement, it is attested by 1540s, borrowed from Luther. The movement began as a bid to reform doctrines and practices of the Church of Rome.
A religious movement in the sixteenth century that began as an attempted reform of the Roman Catholic Church but resulted in the founding of Protestant churches separate from it. Some of the leaders of the Reformation were Martin Luther, John Calvin, and John Knox. The Reformation was established in England after King Henry VIII declared himself head of the Christian Church in that country.