Seen on the dashboard. Can't currently find a URL. May be user specific (IE, if you have an active Gold membership, this probably won't show on your Xbox.)
Whilst buying a 12 Month sub would bring the price down per month, this seems a pretty sweet deal for anyone unable to afford that option.
No obligation to continue the auto-renew. I usually cancel about 2 weeks before MS take another payment.
Offer doesn't seem to have a closing date described.