Another logic riddle

[lastchance]

So, there are 20 monks. They only see each other three times a day, during breakfast, lunch, and dinner, where they sit around in a big circle facing everyone else.

Otherwise, they're in meditation.

These monks believe that being blue-eyed is a sin, and any blue-eyed monk must leave. They are also very logical, and all think at a very high level.

At breakfast, the monks welcome a stranger. At the beginning of the meal, he says "some of you are blue-eyed."

Two days later, not a blue-eyed monk is to be found at lunch, while every other monk stays.

How many blue-eyed monks are there? And how do they know to leave?