Also known as the Garden of Europe, Keukenhof (Lisse, South Holland) is one of the largest spring flower gardens in the world. Approximately 7 million tulips, daffodils and hyacinths are planted annually in the park, which covers an area of 32 hectares (79 acres).
Keukenhof is open annually from mid-March to mid-May. The best time to view the tulips is around mid-April, depending on the weather. I visited in late April 2013 and the tulips were not quite at their prime yet.
For more details about getting to Keukenhof, see here.