Grammatically, the term functions as a noun phrase. The core of the phrase is the noun "shootout," which identifies the type of event. The words "Patriots Day" act as a compound adjective or a noun adjunct, modifying "shootout" to specify its context and connection to a particular series of events.
The phrase specifically refers to the violent firefight between law enforcement and the Boston Marathon bombing perpetrators, Tamerlan and Dzhokhar Tsarnaev. This confrontation occurred in Watertown, Massachusetts, in the early hours of April 19, 2013, four days after the bombing on Patriots' Day. The shootout resulted in the death of Tamerlan Tsarnaev, the severe injury of an MBTA Transit Police officer, and the subsequent escape of Dzhokhar Tsarnaev, which prompted a city-wide manhunt and lockdown.
In application, using this term immediately contextualizes the firefight as the culmination of the manhunt that followed the 2013 Boston Marathon terrorist attack. It is not a generic term but a specific designator for the event that effectively ended the perpetrators' ability to act together and transitioned the crisis into its final phase: the search for and capture of the surviving suspect.
