Nice little anecdote about the 2002 event (does not mention the timing)
From the Worcester Telegram
On May 18, 2002, northern Worcester County saw a small amount of heavy, wet snow. The only official National Weather Service report of that storm was from Ashburnham, where a weather spotter recorded 2.5 inches. There was an unofficial report of 4 inches in Templeton, and heavy snow collapsed tents at the Henshaw Farm - where 20 Confederate and 40 Union Civil War re-enactors fought the battle of the day with snowballs.