1. Part of the marathon was dedicated to Newtown victims.
2. The history of violence on Patriots' Day.
Here's Wolf Blitzer discussing it:
3. The Waco siege ended on Patriots' Day.
4. Contrary to some claims on Twitter, the Oklahoma City bombing did not take place on Patriots' Day.
But:
Bomber Timothy McVeigh's letter and explanation on that here.
5. There's rampant speculation — but so far, only speculation — that the explosion could be either an al-Qaeda attack or a "right-wing extremist" assault.
CNN's senior national security said investigators should pay attention to what kind of explosives are used: "If it was hydrogen peroxide, this is a signature of al-Qaeda. If it was more conventional explosives, which are much harder to get ahold of now, that might be some other kind of right-wing extremists."