The flags are part of a memorial for MIT Police Office Sean Collier, who was killed by the Tsarnaev brothers on April 18, three days after their bombing at the Boston Marathon finish line.
Last year I watched the fireworks from the corner of Prospect Street and Webster Avenue near Union Square, only 2 blocks from what we have since learned was Al Qaeda in Cambridge headquarters on Norfolk Street.
It's said the Tsarnaevs' original plan was to hit the Boston Esplanade crowd during the fireworks. In the dark, with the cover of the fireworks, they might well have killed even more people and gotten clean away with it. This year security was tight and crowds are reported to be lower, but that may have been because it was a very hot muggy day.
In Gettysburg, Pennsylvania they reenacted the Civil War battle on its 150th anniversary. There's an extra verse to The Star-Spangled Banner that was written during the Civil War that seems somehow appropriate both to what happened in Gettysburg in July 1863 and what happened in Boston in April 2013:
When our land is illumined with liberty's smile,
If a foe from within strikes a blow at her glory,
Down, down with the traitor that tries to defile
The flag of the stars, and the page of her story!
By the millions unchained,
Who their birthright have gained
We will keep her bright blazon forever unstained;
And the star-spangled banner in triumph shall wave,
While the land of the free is the home of the brave.