The decision to divide Bengal into eastern and western regions was known as the Partition of Bengal.
The Viceroy of India, Lord Curzon, announced the division on July 19, 1905, and it went into effect on October 16, 1905.
In Bengal, it was commemorated as a day of sorrow.
Final answer:
Hence, October 16, 1905 is regarded as a day of sorrow in the history of Indian National Movement.