The Brimfield Flea Market http://www.brimfield.com/ is held 3 times a year each for a week in Brimfield Ma.. right on Route 20.
Area affected starts At: N 42° 06.849 W 72° 13.590 through N 42° 07.024 W 72° 11.860
all on Route 20...
Route 20 Westbound is traditionally busy from Sturbridge to Brimfield during those times as well.
I find it easier to use the Mass Pike ( RT 90 ) during those times.