From the official Arduino site:
Arduino Day is a worldwide celebration of Arduino’s first 10 years. It's 24 hours full of events – both official and independent, anywhere around the world – where people interested in Arduino can meet, share their experiences, and learn more.
Who can participate?
Arduino invites all Arduino user groups, makerspaces, hackerspaces, fablabs, associations, teachers, pros, and newbies to participate. Let’s make this the biggest birthday party yet!
Arduino Day is on the 29th of March. What can we do to join in on the worldwide event?