Alright, upon review I see that the Loeb edition mentions the possibility that Lucretius died on October 15th, on Virgil's 17th birthday (the very day he assumed the toga virilis). The editor cites "4th century grammarian Donatus, probably following Seutonius", while remaining himself skeptical of the connection. There's no mention of the birth date.
There's certainly no harm in picking a date to honor him, and we don't have any other candidates! The rediscovery of the manuscript by Poggio was in January of 1417, but no day is known.