The Diet of Worms begins, lasting until May 25.

The Diet of Worms of 1521 was an imperial diet of the Holy Roman Empire called by Emperor Charles V and conducted in the Imperial Free City of Worms. Martin Luther was summoned to the diet in order to renounce or reaffirm his views in response to a Papal bull of Pope Leo X. In answer to questioning, he defended the views that had been criticized and refused to recant them. At the end of the diet, the Emperor issued the Edict of Worms, a decree which condemned Luther as "a notorious heretic" and banned citizens of the Empire from propagating his ideas. Although the Reformation is usually considered to have begun in 1517, this edict is the first overt schism associated with it.

May 25 is the 145th day of the year in the Gregorian calendar; 220 days remain until the end of the year.

January 28 is the 28th day of the year in the Gregorian calendar; 337 days remain until the end of the year.