Searching SAS, I found seats departing BOS on Sunday, September 1, returning from HEL on Monday, September 9.
Other dates are also available for travel beginning at the end of August, through most of November.
This Go Light fare includes one carry-on bag, weighing 18 lbs or less. Checked baggage is $40 per piece, per flight.
For $451 roundtrip, you can purchase a standard Economy fare (SAS Go Smart), which includes a full-size carry-on bag, standard advanced seat selection, and allows ticket changes/upgrades for a fee.
See our fare details for booking info.
Need a room in Helsinki? Here's what's available for the above dates on Hotelwatchdog.