5:57 p.m. EDT April 15, 2013  Marathon Day, the city's 'big party,' is upended by powerful blasts. Now, the whys begin. n the Athens of America, on a holiday that marks the beginning of U.S. independence, at the sporting event almost every Bostonian looks forward to, on the day that winter unofficially becomes spring, blasts shattered another American community's sense of safety.

"We've been a calm island in a sea of trouble,'' said Thomas Whalen, a political historian at Boston University. "We think of this as something that happens somewhere else. And now it's happened here.''

And it happened on Marathon Day, which has one of the world's greatest road races, and on Patriots' Day, which marks the first shots fired by the Minutemen at the British at the battles of Lexington and Concord.

And at a sporting event, the kind that had been so successfully policed and secured since the 9/11 attacks. Aside from the bombing at the 1996 Atlanta Olympics, you have to look to fiction — notably the Super Bowl plot in Black Sunday — for such a nightmare.

"It's a day with a festival atmosphere. It's not a day when anybody would ever think of anything like this happening,'' said Larry DiCara, a former member of the City Council.

The day started out well as the Red Sox beat Tampa Bay at Fenway Park in a game that, as always on Patriots' Day, started early, so fans could drift over to Copley Square to catch the end of the marathon.

Ben Beach was going for the record for consecutive Boston Marathon runs.

Out at the city limits, on the downside of Heartbreak Hill, Boston College students occupied the median strip of Commonwealth Avenue to cheer on the weary runners.

For an event so big — tens of thousands of runners, hundreds of thousands of spectators — it always seemed so safe. "My daughter is 15, and I'd think nothing of her going downtown for the end of the race,'' DiCara said. "It's more than just a race. It's a day when kids come home for the long weekend, when families get together and go to the race. It's one big party. It's not a day when you get much work done in Boston.''

Although two of the jetliners hijacked by terrorists on 9/11 departed from Boston's Logan Airport, the city has been relatively free of violence and terrorism jitters. "That's what makes this all the more striking,'' DiCara said.

Whalen said that if the explosions were caused by bombs, "it would make sense as terrorism — if you want to do maximum psychological damage. This is a day when you bring the kids out, everyone has a great time. No one worries. This might kill that.''

For the young people of greater Boston, Marathon Day is a rite of passage, when they pile onto the T with their pals and head to Copley Square. They realize they've arrived someplace, because the square is defined by some of the great monuments of world architecture — McKim, Mead and White's Boston Public Library and H.H. Richardson's Trinity Church.

Whalen thought about those high school kids who'd made the trek Monday from places such as Jamaica Plain and Chestnut Hill and Revere: "What a brutal coming of age.''