Editor’s note: Dan Kennedy is an assistant professor of journalism at Northeastern University in Boston and the author of the forthcoming book “The Wired City: Reimagining Journalism and Civic Life in the Post-Newspaper Age” (University of Massachusetts). He blogs at Media Nation.

I was going through my Twitter feed Monday morning when I came across this: “Happy greatest day of the year, #Boston!” And so it is. Or was, until about 2:50 p.m., when explosions near the finish line of the Boston Marathon transformed a celebration into a scene of carnage.

What matters now, needless to say, are the victims — the dead, the injured and their families and friends. But if you are looking for some insight into Boston at this horrible moment, it helps to understand why our marathon matters and where it fits into our civic psyche. Why it was, until Monday, our greatest day of the year.