There's a list of all of Congresswoman Niki Tsongas' townhalls on the C4L region blog - http://www.campaignforliberty.com/usa/MA/5/

Niki Tsongas healthcare townhall schedule:

Chelmsford, MA - August 8th, 10AM to 12PM - Chelmsford Town Hall 50 Billerica Road

Hudson, MA - August 11th, 5PM to 7PM - Super Stop & Shop Grocery Store 10 Technology Drive (at Route 85)

Lawrence, MA - August 18th, 5PM to 7PM - Market Basket Grocery Store 700 Essex Street 

http://tsongas.house.gov/index.cfm?sectionid=18§iontree=4,18