I've never taken seasonale, but most birth control pills are effective after 7 days. To say they are "85% effective" after one month sounds pretty bogus to me. I've never heard that before. If you read the leaflet that came with your pills, it should tell you how long you'll need to use a back-up when first starting on them. My leaflet (ortho tri-cyclen lo) says to use a back-up only for the first 7 days. After one month, I would assume you'd be totally fine. Best of luck and congratulations on your upcoming wedding!