It could be that most of the seats for the 18th are already booked and the rest are being sold at at premium. It tends to be cheaper the earlier you book and gets more expensive the nearer you get to departure date because the plane is filling up.
That's only my experience. Perhaps someone else knows the real reason behind it. It's a bit of a dodgy practice whichever way you look at it. It should only cost x amount to fly from A to B whenever you book as, theoretically, it's the same distance to get there no matter what date you book on but, hey, that's business.
