At which of the congres session …

Gandhi had announced the launching of the Non Cooperation Movement in August 1920. The Indian National Congress met in September at Calcutta to pass the resolution of the Non Cooperation Movement. But many Congressmen were concerned with the proposals of the movement. They feared that the movement would lead to viloence. From September to December there was an intense tussle within the Congress. In December 1920, Congress met once again in the Nagpur Session. It was finally in this session that a compromise was worked out among the supporters and opponents of the movement and Non Cooperation Movement was finally adopted.
So, Gandhi wished to pass the resolution of the Non Cooperation movement in the Calcutta session but due to opposition he was able to get it passed in the Nagpur session in 1920.