The crew filmed the Boylston Street (in Boston) scenes (marathon finish line and scene of crime) on the South Shore of Massachusetts in a new residential/commercial development called Southfield, which was originally the Naval Air Station South Weymouth. This airbase was an operational United States Navy airfield from 1942 to 1997 in South Weymouth, Massachusetts. It was first established as a regular Navy blimp base during World War II.
Was originally supposed to be about Boston Police Commissioner Ed Davis' experiences during the incident until it was merged with a proposed third movie Boston Strong to give a more general overview of the incident.
The trivia items below may give away important plot points.
The title is derived from the Massachusetts state holiday that commemorates the Battles of Lexington and Concord on April 19, 1775. It's always observed on a Monday, and the Boston Marathon is held that day. Tamerlan Tsarnaev was killed, and his brother Dzokhar captured after a shootout with police, on April 19, 2013, the 238th anniversary of those battles.
The FBI designated the MBTA Transit Police SWAT Team to pull Tsarenaev off of the boat and take him into custody. The handcuffs that were used have been preserved at Transit Police Headquarters in Boston. The SWAT Team secens were edited from the film.