As per the Jyotisha Shastram, the day on which Rohini nakshatra is associated with Krishna paksha Ashtrami is called Jayanthi day. Hence, Jayanthi means it refers to Sri Jayanthi only. If so, are there any specific names for the days on which Perumal incarnated as Varaha, Narasimha, etc.? Why do we call even those days Jayanthi?

Only that oil/ghee (Nei/eNNai) which is extracted from Sesame (El) is called eNNai, which we call as ElleNNai. These days it is common to call anything that is oily, eNNai; hence, we have used unique identifiers to name specific types of oil – thEngAi eNNai, AmaNakku eNNai, vEppeNNai, etc. Without using any Identifier, we call ElleNNai as “nalla eNNai”.
Similarly, Sri Jayanthi is Bhagavan Sri Krishna’s day of incarnation. As Sri Jayanthi refers to the day on which Emperuman incarnated, days of other incarnations are called Matsya Jayanthi, Varaha Jayanthi, Nrisimha Jayanthi, etc. We can day on which Krishna incarnated as Sri Jayanthi