Vesak is a Buddhist holiday celebrating the Buddha's birth, and is celebrated on various days in the Spring, depending on the country. In Japan, it is celebrated on April 8th each year. In many other countries, is is celebrated on the first full moon day in the fourth lunar month of the year, which in 2014 falls on May 13th/14th for those in accord with the Chinese calendar. In the Tibetan calendar, which in 2014 differs from the Chinese calendar by one month, Vesak falls on June 13th.
Vesak also goes by different names in different countries - Visakah Puja (India), Phat Dan (Vietnam), and Saga Dawa (Tibet), just to name a few. It is the largest celebration of the year within many Buddhist cultures, and in some countries combines a celebration of the Buddha's birth, enlightenment, and death. In other traditions, Vesak is primarily a celebration of the Buddha's birth, and his enlightenment and paranirvana (death) is celebrated separately.
Vesak is celebrated differently in each culture and branch of Buddhism, but the emphasis is always on the life and lessons of the Buddha himself. A common practice is for Buddhist practitioners - both lay and monastic - to gather in the early morning of Vesak and honor the three refuges or 'jewels' - the Buddha, dharma, and sangha - through songs and scripture. Monks, lamas, or priests will usually give dharma talks on the Buddha's life and teachings.
In some traditions, Buddhists will renew their commitment to the five precepts of Buddhism - to abstain from killing, stealing, sexual misconduct, false speech, and intoxication. A review of the Four Noble Truths, the Buddha's first teaching, is also common. At temples and monasteries, opportunities to make offerings to a Buddha statue or image are made available, and offerings to the poor and other needy causes are common as well, as a way of honoring the Buddha's message of compassion.
In some countries such as Sri Lanka, Vesak is part of a week-long celebration, during which the consumption of alcohol and meat is prohibited, and various rituals and festivals are held each day. In Tibetan culture, Saga Dawan festivals are also quite elaborate, with celebrations throughout the month. Prayers issued during this time are believed to have a special significance, and prayer flags (such as those shown above) are hung.
The main purpose of Vesak is to honor the teachings of the Buddha, and to renew one's commitment to practicing them.