When is the best time to go to the Keukenhof Gardens?

When it is open. March 21-May 20 for 2013. http://www.keukenhof.nl/

As for an exact date, I suggest keeping your options open because the weather is so variable in Holland. The flowers fold up at night and come out in the sunshine. Pick a warm sunny afternoon for best viewing. Here is my photolog of a visit five years ago, http://tinyurl.com/y8hltjl.

Wonderful photos - I wanna go there!

Mid April is usually high season at Keukenhof. If you are around Amsterdam around April 30 then you can also enjoy queen's day, which is a huge and fun celebration that takes place in the streets and canals all over the city.

We were there two years ago a few days after it opened. There was little blooming in teh gardens, but some things were in flower. Of course the inside exhibitions were fully open, and quite gorgeous.

The gardens are always lovely, no matter what. Mid-April is probably peak time, but I would go at any time.