Ah, well it's celebrated differently in different places.
Details are
here or
here. Lay Buddhists use it as a time to rededicate themselves the Buddha's teachings and to practice, like a New Year's resolution.
In Japan (which I'm most familiar with), it's called Hana Matsuri (flower Festival, as it's tied in with cherry blossom viewing), and people go to temples to pour sweet tea on a statue of baby Buddha. When I was there I did it at Kiyomizu Temple, and it looked similar to
this one.