Mid March to mid April is the peak season.
Netherlands is a great place to visit. here are some tps for using the pubic transport system (safely)
Phil at the safety hub about 6 years ago
It's hard to say what the best time is. Depending on a cold or warm winter tulips bloom earlier or later. The Keukenhof is opened from March 21, 2013 until may 20, 2013 and there are always plenty of tulips to see here.
For the tulips in the field your best option would be 2nd week of April until 1st week of May.
Private tours through the tulip fields and to The Keukenhof are offered by www.holland-is.com almost 6 years ago