General fact: 21 is the number of demands which were sent to the Chinese government by the Japanese government of Okuma Shigenobu in 1915.
Mathematical fact: 21 is a repdigit in base 4 (111).
Trivia: 21 is the number of points required to win a game in badminton and table tennis (before 2001).
Historical date: January 21st is the day in 1893 that the Tati Concessions Land, formerly part of Matabeleland, is formally annexed to the Bechuanaland Protectorate, now Botswana.