I had grown up always believing (and assuming) that Vesak was always the May full moon. Though I have not celebrated it in over 50-years I still think of the May full moon as Vesak. That is why I did my customary, annual Vesal ‘card’ yesterday. Since I know that different, mainly non-Theravadan, countries celebrate vesak on dates different to Sri Lanka I typically put the Sri Lankan date on my ‘card’ to avoid confusion. So, yesterday, I put May 29, 2018.
When I posted my ‘card’ on Facebook two Sri Lankans pointed out that this year Sri Lanka celebrated vesak on April 29, 2018, though some plan to also do so on May 29. Initially I was shocked and upset because I hate to see long held traditions disrupted. I also found the above post which confirmed my initial view.
But, after some research I came to realize that there is some logic to the April 29, 2018 decision — made, of course, by Sri Lanka’s charlatan, often discredited astrologers.
It all has to do with trying to map lunar cycles into the Roman solar calendar — given that vesak is entirely a full moon related event. It eventually comes down to when the full moons fall relative to a given year. April 29th was a LATE full moon for April. That was the problem.
If there are two full moons in May Sri Lanka, traditionally, celebrated vesak on the FIRST of these. A full moon on April 29 was an analogous situation.