Mid-March to mid-May approximately.
One of the things Holland is famous for, is its flowers, particularly bulb flowers. And of these, the tulip is obviously the most famous, but certainly not the only one.
Flower bulbs are one of the main export products of Holland. After being auctioned at the flower auction company in Aalsmeer (the largest in the world!), they are shipped world-wide.
The production of flower bulbs leads to beautifully blossoming fields in many colours. The fields are in bloom roughly from mid-March to mid-May.
Where exactly the bulbs are grown changes every year, due to biological technical reasons. Also, flower bulbs are grown in many parts of the Netherlands, but the best-known part is around the places Haarlem, Lisse, Sassenheim - not far from Amsterdam.
Another well-known area is more North of Amsterdam, around places like Anna Paulowna, 't Zand and Breezand.
Probably the best way of seeing the fields is driving around by car - if you go to one of these areas, you can't really miss the flower fields.
Another way is taking a guided tour from Amsterdam. There are several companies that offer tours, mostly by bus. The tours might be combined with a visit of the Keukenhof, the world-famous flower gardens near the village of Lisse.
Flower field and windmill