Vesak Day celebrates the Buddha's birth and enlightenment and Buddhists celebrate the day around the world. According to the the Singapore Straits Times the date changes depending on the lunar calendar but usually happens in May. This year, the date is today, May 13.
Celebrations often include prayers and offerings of candles and flowers. In addition, notes the Straits Times, some celebrations include the release of caged birds and animals because it symbolizes liberation. And in Singapore, Vesak Day is a public holiday.
The Huffington Post has some great pictures of the celebration including the one above. And here is a calendar showing Vesak Day for the next few years.