If the calendar imposes no restrictions on you, then mid to late April is the best time to see Dutch tulips (assuming no weird climactic conditions in the preceding months). Usually, the week of April 20 is absolutely fantastic.
Most tulip farmers keep a field in bloom for about three weeks – sometimes a bit shorter. There are always early blooming fields and late blooming ones. Additionally, there are a lot of hyacinth fields too and these bloom in early April. (And for the record, hyacinth fields are just as spectacular as tulip fields, but with the added bonus of being particularly aromatic).
This means that you effectively have a five week window where you can visit the Netherlands and be almost guaranteed spectacular flower sights.
To appreciate these fields and sights you either need to book a tour with a specialist tour operator such as ourselves or have your own car and also some local insight as to where to find the fields.
If this is not you, and instead you’re just coming to Keukenhof then the same window more or less applies. Keukenhof is actually open from late March to mid May. However, at either end of this window the Keukenhof is not at its best. There are simply considerably less flowers in bloom. Some people don’t mind this and others find it disappointing. So, as a general rule of thumb, you should be aiming for between April – May 7.
If you’re visiting in mid May, there usually is a tulips field or two to be seen somewhere. However you will need a local guide who knows where to go.
If we had to nominate one day to plan a tulips trip around it’d be April 20 (somewhat randomly). It’s usually peak season then. However, in the event of an early or late bloom there is ample room on either side.