Execute Plugin init function one time
Sessions are initialized by Craft, and plugins are loaded/called after that, so you'll have to assume the session is already up.

Not sure about your use case, but here is how you could execute code once per session:

if(!craft()-&gt;httpSession-&gt;get('initialized'))
{
    /*
    Your code executed once per session goes here
    */

    craft()-&gt;httpSession-&gt;add('initialized', true);
}