I have had a hub of the day once. Mine was for a Photoshop tutorial on restoring old photos digitally. A hub of the day is chosen by HubPages staff and it is determined by how interesting the subject is, how well it is laid out, proper grammar/spelling etc.
there are no "set rules" for how to get a hub of the day accolade. Most hubs that are hubs of the day are the ones you can tell the writer took a great deal of time and care in creating. Many have unique videos or photos accompanying them as well.