December 24, 1851 was the date of the worst fire in the history of the Library of Congress. It lost two-thirds of its books, or around 35,000 volumes.
Fires are not uncommon reasons for destruction of libraries, unfortunately, even in modern days. The Norwich Central Library in August 1994 was considered the worst British library fire in living memory; it destroyed the entire building.
The fire at the Library of Alexandria in ancient times is probably the most famous, but no one knows if it actually happened. Some say Julius Caesar burned the library in 48 BCE; other evidence suggests it was destroyed later, in the succeeding centuries or during the invasion of Arab armies in 642 CE. Perhaps we’ll never know.
The typo conngress* retrieves 13 records from WorldCat; not quite a disaster of library-burning proportions, but good to clean up all the same.
(Photo of the dome in the Main Reading Room of the Library of Congress from loc.gov)