The Twentieth as a commemoration / assembly date was established by the ancient Epicureans in memory of Epicurus and Metrodorus.
The background of this event is as follows:
From the will of Epicurus in Diogenes Laertius:
"The income of the property left by me to Amynomachus and Timocrates shall be divided by them as far as possible, with the advice of Hermarchus, for the offerings in honor of my father and mother and brothers, and for the customary celebration of my birthday every year on the tenth of Gamelion, and likewise for the assembly of my disciples which takes place on the twentieth of each month, having been established in recollection of myself and Metrodorus. Let them also keep the day of my brothers in Poseideon and the day of Polyaenus in Metageitmon, as I have done myself."
Modern Articles Devoted to Observation of the Twentieth:
"Happy Twentieth" - Society of Epicurus
"The Celebration of the Twentieth" - Society of Epicurus
"Twentieth Celebration Posts" - NewEpicurean.com