Direct Action Day (16 August 1946), also known as the Great Calcutta Killings, was a day of widespread communal rioting between Muslims and Hindus in the city of Calcutta (now known as Kolkata) in the Bengal province of British India. The day also marked the start of what is known as The Week of the Long Knives.

The 'Direct Action' was announced by the Muslim League Council to show the strength of Muslim feelings towards its demand for an "autonomous and sovereign" Pakistan. The Action resulted in the worst communal riots that British India had seen.

The Muslim League and the Indian National Congress were the two largest political parties in the Constituent Assembly of India in the 1940s. The Muslim League had demanded, since its 1940 Lahore Resolution, that the Muslim-majority areas of India in the northwest and the east, should be constituted as 'independent states'. The 1946 Cabinet Mission to India for planning of the transfer of power from the British Raj to the Indian leadership proposed a three-tier structure: a centre, groups of provinces, and provinces. The "groups of provinces" were meant to accommodate the Muslim League demand. Both the Muslim League and Congress in principle accepted the Cabinet Mission's plan. However, Muslim League suspected that Congress's acceptance was insincere.

Consequently, in July 1946, it withdrew its agreement to the plan and announced a general strike (hartal) on 16 August, terming it as Direct Action Day, to assert its demand for a separate Muslim homeland.

Against a backdrop of communal tension, the protest triggered massive riots in Calcutta. More than 4,000 people lost their lives and 100,000 residents were left homeless in Calcutta within 72 hours. This violence sparked off further religious riots in the surrounding regions of Noakhali, Bihar, United Provinces (modern Uttar Pradesh), Punjab, and the North Western Frontier Province. These events sowed the seeds for the eventual Partition of India.