I am using My Canon 6D to take photography for an event running from 7-12. Will my battery die before this or will I be good with one battery the whole night?
It is the standard and only battery Canon offers for the 5D Mark II. I use it in both the 5DmkII and the 7D. Not much to say other than it works as advertised and won't void your camera's warranty should something go wrong. If you need longer battery life, get the BG-E6 battery grip for the 5DmkII and run two batteries instead of one.
It all depends on how many photos you take, what proportion will use flash, and how long the screen is on. Those are the main things that use up camera batteries. If you aren't sure how the camera's battery performs under heavy use, bring a second pre-charged battery. If you don't have one, buy one and save the receipt; you can always return it after the event is over.
