If you want to see the tulips in Holland the best time is from the end of March until the second week of May the flowers in the bulb fields bloom. Of course it depends on the weather.
Keukenhof is one of the world's largest flower gardens. It is situated in Lisse. It opens from mittle march till mittle May.
It gets really crowded so go there as soon as you can. It opens from 08.00 to 19.30, at 10 is already busy.
What we did is at 10:30 rent a bike (you can take them out side of the park, 10€ for a day open from 9:30 to 19:30) and go around some fields, you can make different routes between 5 to 25 km.