Wesak or Vesak is not a religion, it is holiday or celebration in the Buddhist calendar. it is often described as Buddha's Birthday in the West, but it involves several other facets of the life of Gautama Buddha.

Wesak is celebrated during the months of Aril or May on the Gregorian calander, by that calendar it is mobile event like Easter. The exact date varies as it is calculated on the lunar calendars used in many Asian countries and the Buddhist tradition that the celebrants follow. The date is usually associated with the full moon in one of the lunar months.

Vesak, sometimes anglicised to Wesak, is one of the most commonly performed rituals of public devotions in Buddhism. It is the annual ritual celebrating the Buddha's birth, enlightenment and death.

Vesak is a Buddhist holiday.

Vesak (Wesak), or Visakah Puja, observed on the full moon day in Taurus; so dates vary each year. Wikipedia has a full discussion at http://en.wikipedia.org/wiki/Vesak.

There are a number of festivals for which we consider special. Theses include Vesak, which is the Buddha's birthday, and Bohdi Day, which is the day that the Buddha become enlightened.