As for the best time of the year to visit De Efteling - spring is the quietest time. Summertime means longer operating hours. Winter means some closed rides (the coasters, mostly) but a very nice atmosphere. In general, I don't think it matters too much when you visit.
The best time to visit De Efteling (but you must be lucky with the weather) by far is fall! There will be lots of people but the nature is at its best in the fairy tale forest!
What a beautiful park!
I've been to Holland before, only for a few days on a more extensive European trip, and it's so beautiful there. I'm planning on going back in the next couple of years and this park will have to be on my schedule