The first Gaura Purnima Festival at Mayapur was celebrated on Srila Prabhupada’s order in 1972.

Below are some relevant quotes from Srila Prabhupada about the importance of the Gaura Purnima Festival in Sridham Mayapur.

“I am very anxious to know whether we are going to have our Mayapur function? I want very much to hold this function this year with all of my students, and I ask you to kindly serve me by making this possible. It is a very important day and it will be a great service to Srila Bhaktivinode Thakura and to His son Srila Bhaktisiddhanta Sarasvati Thakura. So please arrange for this program.”
(Letter to: Jayapataka — Bombay, 5 January, 1972)

“Regarding Mayapur festival, if we cannot do anything there, then because there is very good prospect in Africa I may remain there and not return to India. I especially wanted to stay in India for this Mayapur celebration.[…] Such festival should be held very grandly.[…] So I think you should plan that we shall definitely be coming to Mayapur and hold a grand festival there for Lord Caitanya’s Appearance Day celebrating”
“(Letter to: Jayapataka — Jaipur, 21 January, 1972)

“The best time and place is Mayapur on Lord Caitanya’s Appearance Day. That is not only the best for me but for everyone. Mayapur is meant for that. If possible all devotees from all our centers should go for 8 days at that time.”
(Letter to: Tamala Krsna — Bhaktivedanta Manor, 20 July, 1973)

“The members of the International Society for Krishna Consciousness should go to India during the birthday ceremony of Lord Caitanya Mahāprabhu at Māyāpura and perform saṅkīrtana congregationally. This will attract the attention of all the important personalities in India, just as the beauty, bodily luster and saṅkīrtana performance by the associates of Śrī Caitanya Mahāprabhu attracted the attention of Mahārāja Pratāparudra.”
(Śrī Caitanya-caritāmṛta » Madhya-līlā 11.96 » Purport | 1975 Edition)