I don't know when the best time to book is but the best time to VISIT the park is between April and June. That is when the waterfalls are at their peak. Later in the year, certainly by September, whilst you can see waterfalls some of them have dried out completely and the ones that haven't aren't nearly as spectacular. I would try to go at this time of the year if possible. If that isn't possible, Yosemite is still terrific at all times of the year (except winter when some roads are not accessible) and you'll have a great time.

Thanks for replying - We do want to visit Yosemite in Spring but we're going with friends for our mutual wedding anniversaries. So it's got to be September. One day we'll do it. .....

I went to Yosemite at the end of July in 2006. It was absolutely gorgeous (as I'm sure it is at other times of the year) waterfalls were certainly still flowing. It was very hot though during the daytime