Celebrations of Bihar Day was initiated by Nitish Kumars government in 2010. After the decisive battle of Buxar which resulted in Mughal Emperor Shah Alam bestowing the rights of collecting tax (Diwani) to British East India company, the suba of Bihar , Assam, Bengal and Orissa were clubbed as single administrative unit controlled from Calcutta (Kolkata). Due to cultural differences and separate language the newly educated class of Bihar always felt the need of separation of Bihar from Bengal. Sachiddanand Sinha, Maulana Mazrul Haq, Mahesh Narayan, S P Sinha were the important figures who pressed the British to recognize Bihar as a different state from Bengal. Bihar State Convention was organized in 1908 to 1909 to consolidate the Bihari identity separate from Bengal.
At the same time the British were also planning to shift their capital from Calcutta to Delhi. King George Vth announced the shifting of the capital and the central character of Calcutta in the British government ceased to exist. Bihar got separated from Bengal on 22nd March 1912 and obtained its separate identity which was lost for about 150 years. The idea of Bihari identity gets reinforced after the creation of the state, thats why in the year 2010 Nitish Kumar chose this particular date to celebrate Bihar Diwas. This day remains a official holiday in the state.
There are also school of thought who might not like the idea of Bihar Day celebration on the basis of some action approved by the British government which was largely a treacherous colonial government which exploited and looted the natural resources of the country and used the country to create market for its low quality goods in India.