Vesak is celebrated by Buddhists it is the day that marks the
- Birth of Siddharta Gautam ( The day he was born )
- Attainment as Lord Buddha ( The day he got enlightened )
- Lord Buddha’s Parinibbana ( The day he died )
It is a full moon day and it is celebrated by many asian countries.
Buddhists build vesak lanterns ( mainly from bamboo ) to celebrate this
Vesak was declared a holiday in 1999. You may read more about vesak here - https://lanka.com/events/vesak-festival/