Sextilis was the Latin name for the sixth month in the Roman calendar. Sextilis was renamed to August in 8 BC in honor of the first Roman emperor, Augustus Caesar, because it came after the month of July (named after his granduncle, adopted father, and predecessor, the Divine Julius) and of several fortunate events that occurred in his life during this month: the Battle of Pharsalus, the Battle of Actium, and the deaths of Cleopatra and Marc Antony. It also happened to be the month his grandnephew (the future Emperor Claudius) was born, and the month in which Augustus died. The month reputedly has 31 days because Augustus wanted as many days as Julius Caesar's July, but this idea is a medieval mistake. Sextilis had had 31 days since the time of Julius Caesar (see Julian calendar).
info: Wikipedia