Today is the day!
Check out their website for more information.
A lot of locations are doing flashmobs, but Philly is doing Zumba together. The dancing is an empowering way to channel the emotion around the topic, while displaying to the world that women are not going to take it any more.
If you're not attending an event (some locations do have events throughout the weekend), I encourage you to watch one of the many events worldwide on the website. There are live feeds there, and I assume lots of videos will be posted to YouTube later.
Another way to contribute to the cause: call you congress person and remind him or her to vote for VAWA. The Violence Against Women Act made it through the Senate, but we're waiting on Congress. Here's some info here.