I am not surprised if Sunday is fully booked when it comes to flights from MAD. With today being the busiest day at LHR so far this year, I wouldn't be surprised if people have taken a week off and a lot of them are coming back next Sunday. Factor in the usual load of families away for just a weekend, Spanish people going back home for a bit, another load of language students coming over and business people extending their stay by a couple of days, it quickly adds up.