I just mean that I don’t think it has anything to do with Israel’s Independence Day.
I doubt that the bombing had to do with anything other than a large gathering of people on public streets. And a highly publicized gathering, at that. As you said - opportunity.
I think you are right. The date is coincidental. The significance appears to be from bombing the crowd/participants at a high-profile sporting event, (terrorist act), rather than a specific political statement.
If "Tax day" or "Israel's Independence Day" were the significant motivators, the terrorists would likely have chosen target(s) more closely related to those topics--an IRS office or Israeli consulate--rather than the finish line of a footrace.
The act was aimed at rattling the masses, or the devices would likely have been placed where dignitaries would have been more likely to have been injured.
People using explosive devices to protest something tend to remain focused on what they are protesting, and that affects their selection of target(s). These appear to have been shrapnel laden (antipersonnel) devices, aimed at people, not structures.
Because one could reasonably expect the crowd to contain people of all backgrounds and political views, the objective appears to be one of terror, not a specific viewpoint or policy.