from Wikipedia

Yorkshire Day is celebrated on 1 August to promote the historic English county of Yorkshire. It was celebrated in 1975, by the Yorkshire Ridings Society, initially in Beverley, as "protest movement against the Local Government re-organisation of 1974", The date alludes to the Battle of Minden, and also the anniversary of the emancipation of slaves in the British Empire in 1834, for which a Yorkshire MP, William Wilberforce, had campaigned.

The day was already celebrated by The Light Infantry, successors to the King's Own Yorkshire Light Infantry, as Minden Day. Together with five other infantry regiments of the British Army, a rose is permitted to be worn in the headdress. In the case of the Light Infantry, the rose is white.