Answer: Calcutta (now Kolkata).
Why this is important:
- The Special Session of the INC was convened at Calcutta in September 1920.
- It was here that the Congress, under the leadership of Mahatma Gandhi, debated and approved the Non-Cooperation programme (aligned with the Khilafat movement) as a mass strategy against British rule.
- This decision marked a decisive shift from moderate constitutional methods to a nationwide mass movement.
Related follow-up (helps avoid confusion):
- Later the Nagpur Session (December 1920) reorganized Congress on a provincial (linguistic) basis and further consolidated the Non-Cooperation agenda.