This is a complicated calculation which has been much discussed over the years. Our current best discussion of this calculation is on this lexicon page, which is based on the latest discussion thread here which is itself contain in this section of the "Epicurus" subforum. The short answer current consensus thought seems to be as calculated on this table prepared by Nate with this conclusion:
Quote from Nate from the post linked above
I think celebrating Epicurus' Ceremonial Birthday (Gamelion 20) on January 20th is the way to go for anyone using the Julian calendar. Furthermore, it is the case that in 1987, 1998, 2006, 2017, 2025, 2036, and 2063, Gamelion 20 actually did/does fall on January the 20th, so there are a handful of days when Greeks who practiced according to the Attic calendar, and modern Twentiers who practice with the Julian calendar would have simultaneous celebrations.
Kalosyni, I note that while the Panhellenic Symposium of Epicurean Philosophy (PSEP) is held in February, it occurs on different days every year. For example, this year it was held on the 19-20th. In 2017, it was the 10-11th. In 2016, it was the 6-7th. In 2015, it was the 7-8th. These dates do not correspond with a consistent date on the Attic calendar.
Additionally, while the ceremonial celebration of Epicurus Birthday (on mid-Winter Eikas) sometimes falls in February, it falls in January more often than February. According to my calculations, Epicurus' birth year (Year 3 of the 109th Olympiad, which corresponds with Summer 342 BCE to Summer 341 BCE) would host Gamelion 7 on January 11, 341 BCE and would host Gamelion 20 on January 24, 341 BCE). Either way, during the year of Epicurus' birth, both Gamelion 7 and Gamelion 20 fell in the Julian month of January, so I respectfully disagree with the PSEP (and Wikipedia).
I am curious which Attic Year it is that the PSEP considers to be "the first ceremonial celebration of Epicurus' Birthday". It probably was not until 310 BCE or later that the annual celebration of Epicurus birth date (Gamelion 7) was re-oriented toward the celebration of mid-Winter Eikas (Gamelion 20) because this represents the beginning of friendship with the Lampsacus crew who would go on to carry his torch; Gamelion 20 of that year fell on February 11th 310 BCE.