Vesak (Vesākha, Vaiśākha) is widely considered the most important day of the Buddhist calendar. It is a celebration of all three of the main events of Buddha's life: his birth, enlightenment, and paranirvana (his passing away).
The day itself, informally called "Buddha's Birthday," is spent either celebrating with large festivals featuring vegetarian food and animals, or communing in small groups with a focus on meditation. Aside from this, there is a focus on bringing happiness to others who are less fortunate, especially the handicapped and sick. Buddhists will donate cash and the like to charities, as well as spend time decorating and illuminating temples in place of satisfying one's own needs.
Vesākha is celebrated in a number of countries, including Sri Lanka, Singapore, Vietnam, Thailand, Cambodia, Malaysia, and Indonesia. It goes by a few different names, but in each country the premise is the same.
Even if you aren't Buddhist, we could probably all take a lesson from this holiday. Go out and do something nice for someone today! Tell us about it!