Full details from WestJet's site:
Today only, save 30% off travel within Canada when you pay with your MasterCard‡ at westjet.com.*
To redeem this offer, bookings must be made online at westjet.com and a MasterCard credit card must be used to make the purchase. Both the promo code MCH30 and coupon code FKZUOADare required at time of booking on the first page of the booking process. Bookings must be made by December 8, 2010 (11:59 p.m. CT) and travel must occur between January 6 and March 31, 2011.