I have also encountered attestation as to the opposite like on Wikipedia where it says:
Parinirvana Day, or Nirvana Day is a Mahayana Buddhist holiday celebrated in East Asia. By some it is celebrated on 8 February,[citation needed] but by most on the 15 February.[citation needed] It celebrates the day when the Buddha is said to have achieved Parinirvana, or complete Nirvana, upon the death of his physical body.[1]
That [1] citation leads to a hopefully well-researched BBC page that says:
This is a Mahayana Buddhist festival that marks the death of the Buddha. It is also known as Nirvana Day.
It’s not hard though to find parallel attestations on random Internet pages and search results that substantiate the reverse: that it is a Theravāda celebration.
It seems to be something the Internet doesn’t have an easy consensus on. From a different Wikipedia page we have an attestation of the Buddha’s birthday as a “Swānyā Punhi (स्वांया पुन्हि)” , which:
is celebrated on the full moon day of May. The festival is known by various names, Buddha Jayanti, Buddha Purnima, Vaishakh Purnima, Saga Dawa, and Vesak. Purnima means full moon day in Sanskrit. Among the Newars of Nepal, especially from the Shakya clan of Newars, it is of great importance because they consider it as a continuation of the sage of the Śākyas- the clan that Lord Buddha’s family belonged to. Thus, they celebrate the festival which is in their language known as Swānyā Punhi (स्वांया पुन्हि), the full moon day of flowers.[18] The day marks not just the birth of Shakyamuni Gautam Buddha but also the day of his Enlightenment and Mahaparinirvana.
The end has the birth-awakening-parnirvāṇa in-one-celebration/day tradition ascribed to Vajrayāna Buddhists in Nepal, practising in a tradition in cultural sprachbund other Buddhisms, chiefly those in Tibet, at least AFAIK.
The [18] leads to this. It is an archived we page that looks (?) like it might be a newspaper for the Nepalese diaspora in English?