Please allow me to share my opinion. I believe when the Buddha's teachings began taking root outside of India in East and Central Asia (from 4th century CE), it was the Mahayana version which became popular.

The decline of Nikaya Buddhism (Hinayana) in India was fortunately supplemented with the rise of Mahayana Buddhism. Students of Mahayana will tell you that the concept of the Buddha is more expansive than just the historical founder. Mahayana teaches that the Buddha has three bodies, the Dharmakaya (Dharma bodhi), the Sambhogakaya (the transcendent Buddha), and the Nirmanakaya (physical form of the historical Buddha).

Based on this idea itself, some schools such as Pure Land put more emphasis on the the transcendent Buddha, which is encapsulated as Amitabha. Mahayana also emphasise the "bodhicitta" ideal, where each and every one of us has the Buddha seed, where every individual has the potential to attain Buddhahood.

With the above reasoning, there is no necessity to set a definite fixed date to commemorate Buddha day, applied to all Buddhists since the Buddha "did not die". He became "transcendent" and His teachings continues to be transmitted.

The issue with the WFB 1950 resolution lie in the proposal to set a single day for Vesak for all Buddhists to mark the Buddha's birth, enlightenment, and passing into nirvana. There is no evidence in the Pali Tipitaka which states that these events occured on the same day. Moreover, the proposal was skewered towards Nikaya Buddhism, which I believe received lukewarm reactions from Mahayana Buddhist countries.

Another matter which did not receive consensus was on the way full moon was defined. In the Pali text, everyone accepts that the historical Buddha was born on the full moon of Vesakha. While both the Chinese and the Hindus follow the lunisolar calendars, their approach are different.

Chinese lunisolar calendars track more or less the tropical year whereas the Hindu lunisolar calendars track the sidereal year, which, simply means that chinese calendar gives an idea of seasons whereas indian calendar gives an idea of the position among the constellations of the full moon, and difference of meridians. What this means is that the Chinese calculation of its full moon will differ from that of the Hindus.

Which is why this year 2023 is peculiar in that there is a leap year according to the Chinese calendar. So for example some countries will observe Vesak on different months. Malaysia celebrated Vesak yesterday on 4 May, while Singapore will observe it a month later on 2 June (during the full moon of the leap month). In previous years, Malaysia and Singapore always share the same Vesak day.

And yet Malaysia's date is one day off from Buddha Purnima celebrations of the Indian subcontinent, where India, Nepal and Sri Lanka collectively observes the holy day on 5 May. This is a classic case of Malaysia following the Chinese lunisolar full moon day, while the three Indian sub-continental countries used the Hindu lunisolar calendar. Again in previous years, Malaysia's Vesak is usually in sync with that of Sri Lanka.

I believe the reasons laid out above explains in some way why there will never be a common Vesak or Buddha day for all Buddhists. Nevertheless, I also believe that this is a non-issue with Buddhists generally. As long as the Buddha Dharma continues to thrive, and all Buddhists are still able to observe the holy day albeit on different days, everyone should still be happy.

Let us all appreciate the diverse cultures of the people of the Buddhist faiths. Despite different culture, language and tradition, we are still able to bow to the same Buddha and follow his basic teachings. At the end of the day what matters is that we learn the basics of kindness and compassion, and put it to good use