The Boston Marathon always occurs on Patriots' Day, observed each year on the third Monday in April as a commemoration of the Battles of Lexington and Concord. Residents of Massachusetts and Maine celebrate Patriots' Day with re-enactments of those early battles of the American Revolution. But in recent years, Second Amendment activists and anti-government modern-day militia members have tried to co-opt the holiday, which also roughly marks the anniversary of the Oklahoma City bombing.
In 2010, the Daily Beast's John Avlon chronicled an attempt by one militia-affiliated group to host a pro-gun rally in Oklahoma City on Patriots' Day. The organizers eventually rescheduled that event under pressure from local officials, but a separate rally did proceed in Washington, D.C.
Keep in mind though that the Oklahoma City bombing actually took place on April 19, not April 15, which despite falling on Patriots' Day this year is a date more typically associated with Tax Day. There's currently no information about who might be responsible for today's attacks in Boston.