Prabhupada said that it would suffice to celebrate Lord Varahadeva’s appearance by singing the appropriate verse describing him from the song of the ten avataras.
Srila Prabhupada also verbally listed the proper fasting periods to observe for other auspicious days: Lord Nityananda’s, Sri Advaita’s, Lord Balarama’s, and Srimati Radharani’s appearances are all half-day fasts. Lord Nrsimhadeva’s and Lord Ramacandra’s appearance days are full day fasts until sunset while Gaura Purnima is a fast until moon rise.
Tamal Krishna Maharaja inquired about observing the appearance and disappearance days of great Vaisnavas. He described how we observe a
half-day fast for our immediate predecessor acaryas. He wanted to know if we should also do the same for personalities like Srila Narottama dasa and others like him. Prabhupada said yes, if it is possible. But he added that if devotees are engaged in preaching work, they may not. He said that the main thing is to sing some songs of praise and perform kirtana.
From Hari Sauri Prabhu’s “A Transcendental Diary”