How to create a "sub domaine" site?

We have a WordPress themed website and need to create the next event "Mind Control & Super Soldiers: Summit II" on the same domain for marketing purposes.
Can we move the entire first events website content into a sub folder called, for example:{?page_id=153, etc .} and access it through a "past events" button on the new, main site?
And how would we do this? Muti Sites? If Muti Site is the answer, where can we learn how to move this site correctly "down the tree" without losing any data?
Or can this be done within most themes?
Thank you!