This is the celebration of Buddha's birth. For Theravada Buddhists, it is also the celebration of Buddha's enlightenment and death. During this celebration, statues of Buddha are decorated. Offerings are taken to monasteries, and sometimes there are fireworks.
This is a time to meditate and study. In Buddha's time, it was during the rainy season which meant it was hard to travel and teach so it was a good time for meditation and study. During Vassa, a meditation retreat, all Buddhists are supposed to set aside some time for study and meditation. At the end of Vassa, people bring new robes to monks.
Different countries have different Buddhist celebrations. For example in Japan, Buddhists celebrate the flower festival, or Hana Matsuri, to honor Buddha's birthday. Temples are decorated with cherry blossoms and children pour scented tea over statues of the baby Buddha. In India, Buddhists celebrate the Festival of the Sacred Tooth in honor of Buddha's first teaching. One of Buddha's teeth is paraded around in the streets as an expression of this celebration.