I'd read that this is the most popular sight in Spain, and now I really believe it.
http://en.wikipedia.org/wiki/Alhambra
http://www.alhambra-patronato.es/
I'd also read that some tourists just show up in Granada for a visit of a few days, hoping for the best, and aren't able to get in. Imagine how disappointing that would be. So I wanted to book tickets well in advance.
I tried their web-site multiple times in March, but they hadn't made tickets available yet for May (we'd originally planned to visit the Alhambra on Wednesday the 27th, preferably in the late morning), so I wanted to try the site today, April 1st, thinking that April/May would come up all at once.
I logged in at 1 this afternoon my time and all that was left on the 27th were a few tickets at 8:30 am and a few more at 4:30 pm! So I looked at the 28th. There was already nothing at all left for the morning, but there were 6 tickets for 2 pm (the earliest ones you could book in the afternoon), so I bought them fast, and now we have them.
Mind you, technically, it became 1 April in Granada at 3 pm on 31 March in Vancouver. But STILL! I figure that tour groups must swoop these up like crazy the moment they become available. Even at that, you're only allowed 10 tickets at a time, so they must either have very cheap labour on-tap or a computer program that repeatedly returns to buy these admissions by the hundreds!
Anyhow, boy am I glad we booked as early as possible and that we're going to get to see it!