On that same day Steven Colbert is hold a "rival" rally to keep American's afraid and under control of the government.
Many people think this is just an act to get publicity, which I partially agree with, but I think more than just satire is going on here.
This is an attempt to show everyday people (moderates) that these extremists are morons. Both sides throw the most ridiculous claims at their rival party.
Even Colbert's rally is using this to show American's what these extremist groups are doing to us. They are just try to make us afraid so you vote for them, and that is not how an election should work in any developed and civilized country, or any country for that matter.
I urge anyone that has the means to attend one of these marches to do so! You will be helping put American politics back on the correct path. And I'm sure it's going to be a blast!