Thursday marks the 13th anniversary of 9/11. For those looking to pay their respects to the thousands of men and women and children who lost their lives in the crash of United Airlines flight 93 in Pennsylvania, the attack on the Pentagon in Washington D.C. and the World Trade Center in New York City, there are three national recognized sites open to the public. Here is everything you need to know if you plan to visit on Sept. 11:

Flight 93 National Memorial

Flight 93 Memorial President Obama and first lady Michelle Obama visit the Flight 93 memorial in Pennsylvania on the 10-year anniversary of 9/11. Photo: Reuters

Location: 6424 Lincoln Highway, Stoystown, PA 15563

Fee: None.

Hours: On Sept. 11, the memorial will be offering extended hours, opening at 8 a.m. and closing at 7 p.m. The last entry will occur at 6:30 p.m.

Tributes: Guests are permitted to leave small tributes along the Memorial Plaza wall and the Wall of Names. Large tributes, such as wreaths, must be arranged in advance.

Events: The annual observance begins at 9:45 a.m. Thursday, rain or shine. The event is open to the public. The ceremony will begin with the reading of the names of Flight 93’s passengers and crew and the ringing of the Bells of Remembrance. This year’s observance will feature a speech by former Speaker of the House of Representatives Dennis Hastesrt. In honor of the victims of Flight 93, the Congressional Gold Medal, the highest civilian awards offered by the U.S. Congress, will be displayed at the site Sept. 11-14.

Notes: No seating will be provided to guests. Visitors are welcomed to bring their own chairs. Umbrellas are not permitted.

The National 9/11 Pentagon Memorial

Pentagon Memorial A couple stands at a 9/11 memorial in the Washington, D.C., area in 2012. Photo: Reuters/Stringer

Location: 1 N. Rotary Rd, Arlington, VA 22202

Fee: None.

Hours: The Memorial typically operates 24 hours a day, 365 days a year. The memorial will be closed for a private event from 6 a.m. to 11 a.m. Thursday.

Tributes: Visitors are allowed to leave mementos at the Memorial.

Events: Upon the reopening of the memorial at 11 a.m. Thursday, visitors are invited to participate in the Pentagon Memorial Audio Tour. The audio guide, approximately 24-minutes in length, tells the history of the Pentagon and the events surrounding the attack on the west wing of the building on Sept. 11, 2001. To access the recording, call (202) 741-1004 at the entrance of the park.

Notes: Restrooms are open daily from 7 a.m. to 10 p.m. Smoking, eating, drinking (not including water) and pets of any kind are not permitted on the grounds. Photography is only allowed within the Pentagon Memorial, not its surrounding pathways/drop-off areas.

The National 9/11 Memorial

9/11 World Trade Center Memorial Flags line one of the World Trade Centers' National 9/11 Memorial pools during the 12th anniversary of the attacks in 2013. Photo: Reuters

Location: (World Trade Center) Liberty St, New York City, NY 10006

Fee: None.

Hours: The memorial is open daily from 7:30 a.m. to 9 p.m. On Thursday, the 9/11 Memorial & Museum will be open to the public from 6 p.m. to midnight.

Tributes: Leaving mementos in front of the Memorial pools/the bronze Memorial name plates is encouraged. All tributes must be no larger than 8”x17”x19” and are subject to a security screening.

Events: Starting at 8:39 a.m. EDT Thursday, the 9/11 Memorial will hold its annual commemoration ceremony, including the annual reading of the names of the 2,983 victims killed in the attacks. Families of victims and first responders will be given private access to the National Sept. 11 Memorial & Museum until 6 p.m. Thursday when the site will reopen to the public.

Notes: Public restrooms are not available at the Memorial or in its surrounding hotels. Guests are asked not to dispose of anything into the Memorial pools. Photography is permitted.