Lewis & Short

īcas, ădis, f., = εἰκάς (the number twenty), the twentieth day of the month, kept as a festival in honor of Epicurus (because Epicurus was born on the 20th of the month Gamelion), Plin. 35, 2, 2, § 5 (for which: omnibus mensibus vicesimo die lunae dent ad eorum epulas, etc., Cic. Fin. 2, 31, 101).