When did the leaves change color?

I have been watching the leaves to find the exact day on which the leaves began to change color. Last week they were green. Today there is a lot of yellow up there. Was the day on which the change happened Friday or Saturday? Why is it that every year I am surprised by suddenly realizing that the leaves are changing color, even in a year in which I was trying to watch the process?

When did you notice that the leaves had begun changing color?

I posted so many pictures of leaves last year (here, here, here, and here, (and maybe a few other places as well) that I will try to avoid the topic this year.