You can find flights at this price starting in late October through early December. Departure days vary, but you'll have the most luck departing on Monday and Wednesdays, with returns on Thursdays and Fridays. Since this deal is on Icelandair, you can add a free stopover in Reykjavik for a few days on either leg of the trip, free of charge. Two vacations for the price of one!
This is Icelandair's Economy Light fare, which only includes a standard sized carry-on bag plus one personal item. If booking on Icelandair.com, you can get a 20% discount on checked baggage fees during or after booking. Online checked baggage fees start at $76 each way.
Searching on the Icelandair website, I found seats departing BOS on Monday, November 5, and returning from BRU on Friday, November 16, 2018.
Icelandair has tons of great deals out of Logan today, so take a look at our Boston BOS flight deal page.
Need a place to stay in Brussels? Here's a full list of available hotels for the mentioned dates on Hotelwatchdog.