"Recognize Bangladesh Satyagraha" was held by the Bharatiya Jana Sangh for eleven days between August 1-11 in 1971.
The eleven-day rally of 1971 referred by PM Modi was covered in the media reports. The Bharatiya Jana Sangh, led by former prime minister Atal Bihari Vajpayee, organized a rally called ‘Recognize Bangladesh’ in Delhi from August 1 to 11 in 1971. The organization undertook the protest to demand the Indian government’s support in recognizing Bangladesh as an independent country. While some reports also alleged that the rally was in response to the India-USSR Treaty of Friendship and that the Jana Sangh wanted to prove it as a betrayal of Bangladesh.
The rally was reported by the Times of India that said Vajpayee had addressed the gathering on the last day of the protest on August 12, 1971, implying the treaty with the Soviet Union as a conspiracy between Delhi and Moscow to deny recognition to Bangladesh.
Another article from Economic Times featured an image of Vajpayee, captioned as him addressing the mammoth rally in Delhi in August 1971, demanding the immediate recognition of Bangladesh by the Indian government.
The protest videos were also posted by the Associated Press archive, describing it as on August 2, 1971, “Demonstration in New Delhi by members of Jana Sangh party in which six hundred people were arrested.” Another video showed the last day of the rally, August 12, 1971, captioned as ‘Thousands of Jana Sangh party members staging a mass demonstration in New Delhi protesting at Indian government's failure to recognize Bangladesh.’
Therefore, an eleven-day rally was held in August 1971 by the Jana Sangh supporting Bangladesh’s liberation.