Why do some areas in West Bengal celebrate Independence Day on August 18?

Some border areas in West Bengal, mainly in Nadia and Malda districts, celebrate Independence Day on August 18. Their freedom came three days late because a mapping error by the 1947 Boundary Commission, led by Cyril Radcliffe, assigned these densely Hindu populated areas to East Pakistan (Bangladesh). After massive protests, Viceroy Louis Mountbatten ordered redrawing of the partition of Bengal.