Under which Act was Dadra and Nagar Haveli incorporated into the Indian Union?

Dadra and Nagar Haveli Integration Day is observed on August 11, as the territory was incorporated into the Indian Union on August 11, 1961; prior to this, it had been liberated from Portuguese rule on August 2, 1954. It was designated a Union Territory via the 10th Constitutional Amendment Act, 1961, and was merged with Daman and Diu on January 26, 2020, to form the current Union Territory.