Vesak is kind of confusing to a Shin Buddhist. In the Buddhist Churches of America, we celebrate Shakyamuni Buddha’s birthday on April 8 (and Shinran Shonin’s birthday on May 21), so why are we celebrating Shakyamuni’s birthday again in May?!! I addressed this issue in my Dharma Talk at Buddhist Church of Oakland this past weekend:
TLDW : Vesak is the Theravada commemoration of Shakyamuni, the historical Buddha’s birth, enlightenment, and passing away. It is calculated based on the lunar calendar, and thus the date changes each year, like Easter or Lunar New Year. Nishi Hongwanji, on the other hand, calculates the various events based on the solar calendar. I personally think it is important to acknowledge these differences, even within the greater Buddhist tradition. Plus, it looks like Vesak is getting some more traction nationwide…
gyokyo 
Yes, it can be confusing for those not familiar with the different contexts. Many South East countries tend to use Vesak day as Buddha’s birthday. It is a public holiday in Thailand, Singapore, Malaysia etc. Some ethnic Chinese temples however still use the 8th day of the lunar 4th month. Most would use the Vesak day as it is gazetted as a public holiday.
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