The Event Day Management is a service that Ariba offers for your organization. You either accepted it or decline it. If you decline, then the owner/creator of the bidding event would conduct the event management tasks. You don't want to give everyone Event Administrator permission because that would make them a "User" for licensing purposes and most likely put you in an overage situation. The template is typically setup with the event owner in the Event Admin group in the event, so the event owner will inherit the capabilities needed to manage the event.
Please contact support for more information about both choices.