Elderly survivors of the Holocaust and the veterans who helped liberate them are gathering for what could be their last big reunion at the U.S. Holocaust Memorial Museum.

One thousand survivors and World War II vets are coming together with former President Bill Clinton and Nobel Peace Prize laureate and Holocaust activist Elie Wiesel on Monday as the museum marks its 20th anniversary. Organizers chose not to wait for the 25th milestone because many survivors and vets may not be alive in another five years.

Clinton remarked that Washington has many monuments and memorials, "but the Holocaust memorial will be our conscience."

The museum is also launching a campaign to raise $540 million by 2018 to keep the memory of the Holocaust alive and to combat anti-Semitism, Holocaust denial and contemporary genocide.