The main ceremony is in a different place each year. This year it was in my hometown, Santander, and involved a simulated combined air, sea and land amphibious assault in a beach, and a display of army vehicles and other material.
Here is a link to pics of the amphibious assault -and you can see how a Piranha III got stuck in an underwater hole in the sand

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http://cid-20ec43cd1dacddde.skydrive.live.com/browse.aspx/Dia%20de%20las%20fuerzas%20armadas%20|5Santander%202009|6