Ancient sources cite the 10th day of the month of Gamelion as the day of Epicurus' birth. As such, he held a feast in his Garden, and later when his hero cult evolved he was celebrated on that day.
The month of Gamelion is the month of marriage (gamos), situated in the latter part of January and the early part of February. It doesn't fit exactly into our calendar because the Attic calendar in use in ancient Athens was lunar, not solar.
Modern Epicureans have two options to establish a day for celebrating Epicureanism: one would be to stick to the 10th day of Gamelion and count ten days from the January new moon, which is a bit archaic in my view and opinion.
The more practical option is to choose either January 10th or some day in mid January as a day for outreach and education on the legacy of Epicurus. Maybe when there are more Epicureans on the forum, we may be able to have a vote on what is the best way to fix a date for Epicurus' birthday in our calendar: I think it would help to generate awareness of his legacy and more visibility for humanism in general.
Thanks for this information.
There is - as far as I know - no reliable source as to when was Epicurus' birthday in the calendar used at his time, and as calendars have changed often since then, I am not sure if it is possible to calculate correctly any given date from then to today.
But I think that dedicating one day of memorial to someone does not really require it to be his birthday, any arbitrary date can just as well serve the purpose. So I go along with the suggestion made elsewhere to dedicate January 21st to Epicurus, no matter if it happens to be his birthday or not.
So here is my message to him: γεια σου, Επικουρε, και χρονια πολλα.