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The traditions that have defined the Boston Marathon for 118 years — a joyous holiday for the city, thousands of triumphant runners streaking or limping across the finish line — will wait a week.
First, on Tuesday, Boston will pause for one somber day to observe what happened exactly one year earlier, when two bombs detonated seconds apart, shattering tradition, glass and bodies.
"I think it’s going to be a difficult day for a lot of people," Boston Mayor Martin Walsh told NBC News on Monday.
Walsh, Vice President Joe Biden and Gov. Deval Patrick and will speak at a commemoration at a convention center not far from the finish line on Boylston Street.
“Though we all had our lives affected in profoundly different ways, this tribute will show the world again that we stand as one,” Patrick said in a ceremony announcing the event earlier this year.
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They will be joined by first responders and survivors of the marathon, a term that took a new and tragic meaning after the blasts on April 15, 2013.
The service will conclude on Boylston Street, over a half-hour that includes the time the bombs went off, with a moment of silence and a flag-raising. There will be no speakers there.
The marathon itself will be run April 21 — Patriots Day in Boston, which falls on the third Monday of the month and honors the Revolutionary War battles of Lexington and Concord.
About 36,000 people are expected to run, and to be cheered by perhaps a million people along the 26.2-mile course from Hopkinton to Copley Square, in the Back Bay section of Boston.
Extra security measures are in place for this year's run, including additional police officers, surveillance cameras, checkpoints and bans on backpacks, glass containers, costumes and large flags.
But authorities will not stop crowds from gathering at the Boylston Street finish. They have stressed that they want the marathon itself to preserve its atmosphere and celebration.
The tribute on Tuesday will honor the three people who died in the marathon blasts and the more than 260 people who were injured, some gravely.
The solemnity will blend with the expressions of resilience and camaraderie that sprang up as soon as the chaos cleared, most notably with the "Boston Strong" rally cry that extended from Twitter to the Red Sox dugout at Fenway Park.
The tribute will also honor the One Fund Boston, which collected $61 million in the first three months after the bombing and distributed it to victims and their families.
The ceremony begins at noon ET and will be streamed live on NBCNews.com.