Vesakh Festival - in April
...it actually encompasses the birth, enlightenment and passing of the Buddha.
Poson Festival - in June
recalls that the son of the Indian emperor Ashoka, Buddhism in the 3rd Cent. BC brought to Sri Lanka.
Kathina Festival - in October
The traditional Buddhist Kathina Festival is on full moon at the end of the rainy season.
In our time calculation, it is the full moon of the gregorian of month October or November.
The essential element is it to offer new robes for the monks of a monastery, what's especially in Theravada Buddhism is practiced.
The dates of these celebrations vary annually and will be announced.