Why does this article say Bach's birthday was March 31, when many other sources say it was March 21?
Due to the change from the Julian calendar ("Old Style") to the Gregorian calendar ("New Style"), both dates are valid: March 21 in Old Style dates, March 31 in New Style. Wikipedia uses New Style dates, but also includes the Old Style in small print – hence the "(OS 21 March)" notation. See Old Style and New Style dates for more details.