I've received an email from United Airlines, announcing deals on frequent-flyer tickets to 8 major European cities. You must book no later than tomorrow (Saturday, Nov 25). Travel dates are given as Nov 24 - Dec 14 and Jan 8 - Mar 21. I briefly fiddled around with the booking page on united.com and found some reductions extending into April and May (at least), so it's worth checking if you have United miles to use and are planning to travel after Mar 21.
This webpage provides some details; mileage requirements are lower for those with status and/or a United credit card.