April 15th is a coincidence.
This being Tax day, Boston, whatever, is irrelevant.
The reason today was picked was due to the marathon. It was the crowds. Bombs need to be set off in a crowd of people for maximum effect.
Boston is also a coincidence. It's a major city. The group that set this up is America based, east cost. I'd bet they are located somewhere between New England and Virginia. They picked Boston as it was close (they needed to drive, can't fly with a bomb). They picked today due to the large crowds.
There's also much less security at a marathon, compared to parades, speakers, etc.
Best potential, least risk. Coordinated attacks. No statements or demands. Note that these weren't fertilizer bombs, these were actual bombs. Smart, trained people pulled this off. This is professional.
That being said, it probably is a right wing group. Tax day just happened to line up with it, which is a bonus. But yeah, the level of professionalism here implies an ex-military group of at least 2 people. Right wing militias would fit this description.
The bombs were planted several hours earlier, the agents then left the scene. They were probably all home when they went off.
They aren't going to have contact with each other for at least 6 months after this. They probably haven't had contact with each other for at least 6 months before this. This was planned at least a year in advance (I would assume). Given their level of proficiency and planning, it could have been planned much faster.
If I was a betting man, I would say a right wing group pissed at Obama's reelection. Just my opinion.