In Epicurus and His Philosophy, DeWitt proposes a date of February 7th:
Quote"The relevant dates are known with a precision that is uncommon in the lives of great men of ancient times. He was born of Athenian parents on the island of Samos in early February of the year 341 B.C." (36)
Quote"At any rate [...] the date was fixed, not for the anniversary day of his birth, which fell on the seventh, but at the twentieth, the day that marked the final initiations at Eleusis. The twentieth was also sacred to Apollo, which gave it an additional sanctity. Such notoriety eventually attached itself to these monthly memorial gatherings that Epicureans were dubbed 'Twentyers' by way of derision" (51-52)