Members of Alpha Phi Alpha gather for a private ceremony to inaugurate the Martin Luther King Jr. Memorial, named the country’s 395th national park. (BILL O'LEARY/THE WASHINGTON POST)

On Sunday, the 48th anniversary of King’s “I Have a Dream” speech, the NPS formally declared the memorial America’s 395th national park.

“Welcoming this memorial to the National Mall honors a heroic man and a critical chapter in our nation’s march toward a more perfect union,” said Secretary of the Interior Ken Salazar.

This weekend was slated to be a big one for the memorial, with events including a prayer service, concert and special tribute by the Alpha Phi Alpha fraternity.

But as Irene threatened to bombard the Washington area, most memorial events Saturday and Sunday were canceled, including a march for District voting rights, a “jobs and justice” march, several parties and the star-studded dedication Sunday expected to be attended by President Obama, which will be rescheduled for September or October.

For more on the Martin Luther King Jr. Memorial, including a critical review of the site and an interactive guide for getting around, visit The Post’s full coverage page.