Dies Iræ is a Latin hymn describing the day of judgement. The first line is as follows:
Dies iræ, dies illa
Since used in the hymn, the first few notes of Dies Iræ have appeared in a lot of music, especially movie soundtracks, from The Nightmare Before Christmas to It’s a Wonderful Life to Star Wars: A New Hope. Once you hear it, you’ll start noticing it everywhere. This video gives a history of Dies Iræ, from classical music to modern movie soundtracks.