When was the first vesak?

The first Vesak, which commemorates the birth, enlightenment, and death of the Buddha, is believed to have been celebrated shortly after the Buddha's death around the 5th century BCE. However, the formalization of Vesak as a significant Buddhist festival occurred later, with its observance becoming more widespread among Buddhist communities by the 1st century CE. The specific date of the first celebration is not precisely documented, but it has been an important observance in Buddhism for centuries.