Schoenberg was an Austrian composer and creator of the twelve-tone technique. This method of composition ensured that all 12 tones of the chromatic scale would be used equivalently throughout a piece.
For as much he as he liked using all 12 tones, he was terrified of the number 13. His triskaidekaphobia (fear of the number 13) was so intense that he even changed the title of one of his songs so that it wouldn't have thirteen letters.
On Friday, July 13th, 1951 Schoenberg stayed in bed all day, sick with anxiety. This was both because it was Friday the 13th and due to the fact that he was 76 years old (7 + 6 = 13). He died of unknown causes roughly 13 minutes before midnight.
(Sources: 1, 2)