Mafeking was a British held town in South Africa that the Boers sieged during the South African War. This siege went on from 12th October 1899 until the 17th May 1900 when the British held the Boers off until the British Garrison arrived and the town was relieved on 17th May 1900.

So on the 21st May 1900 the departments celebrated the Relief of Mafeking.