What is Rolling Thunder? Memorial Day's motorcycle tribute

Each Memorial Day, thousands of motorcycles descend on Washington, D.C. for the annual Rolling Thunder ride. The event, now in its 25th year, honors those who died or were captured while serving in the military.

On Sunday (May 27), motorcyclists start at the Pentagon and converge on the National Mall to pause for a ceremony at the Lincoln Memorial. Then, on Monday -- Memorial Day -- there are more events planned, including a ceremony at Arlington National Cemetery and another at the Vietnam Veterans Memorial.

Roughly 400,000 bikers were expected to ride on Sunday.

"Todd and I and our family would like to wish you all a happy Memorial Day weekend," tweeted Sarah Palin on Friday (May 25). "We'd especially like to offer our best wishes to our friends in Rolling Thunder who will be taking part in their Ride for Freedom this weekend in D.C. as they do each year to honor our vets and specifically to bring awareness to POW/MIA issues. We are honored to join them last year."

The message came in the wake of a quote from one of Rolling Thunder's organizers who tells MSNBC that Palin was not welcome this year.

Other Memorial Day events in the capital include the annual PBS National Memorial Day Concert hosted by Joe Mantegna and Gary Sinise. Performers include Trace Adkins, Daughtry and "American Idol" runner-up Jessica Sanchez.