People dress in green, there are parades in many cities around the world, and it's a day for celebration and having fun. Here's one of these parades in Dublin, Ireland.
This year, we are lucky to have a special reporter, Srta Becquer, in the streets of New York sending us her twitts of the day:
Thank you, Srta Becquer, it's great having you there today!
And here are some pictures of children in green at school: