Here’s the deal. My darling Agent Foxtrot is thoroughly obsessed with politics, will talk your ear off about Pubbies and Demmies and so forth, but despite having lived an hour from D.C. his entire life, he has never attended one political demonstration. I attended two last year (March for Women’s Lives in D.C. and a demonstration against police brutality in Philly) and I’m determined to take him along to another.
The problem is I can never seem to find out about them far enough ahead of time to go. I can’t find a website or anything that lists upcoming marches and demonstrations. The D.C. tourism website doesn’t have anything like that included in their “Events” calendar. I know there’s a Prison Reform March in August, and I’m planning to go, but other than that I’ve found zilch. Surely there’s a website or something. I just don’t know where to look. Do any Dopers have any suggestions?
Hmm… If I was in your shoes, I’d found out what kind of permit was necessary to hold a march (perhaps by calling the non-emergency police number, or city hall), then go to or call the office that takes care of the permits and see if there’s public records available.
Unfortunately, I can’t find a listing of permits issued for protests… but I do have a listing of authorized street closings in the city. Since any demonstrations would likely involve the closing of streets, they would be listed here. Unfortunately, it appears to only list the closings with a week’s notice or so.
Next year, there will be a month-long march right after February. 
I just called the Metro Police, and they said that they don’t have a list available to the public, except for major marches or protests involving street closings, which would be announced on their website.
Your best bet might just be to go to websites of major protest groups, and check there, periodically, especially on certain dates that might be important to those groups (anti-globalization people on times the World Bank and IMF have major meetings, pro-choice and pro-life people on the anniversary of Roe v. Wade, etc). Or just go to Lafeyette Park and talk to the anti-nuke people who hang out in front of the White House. All the excitement of political protest without the crowds.