I have been to the Keukenhof garden in Lisse outside Amsterdam a couple of times. The first time was in early April when the giant blocks of colour created by mass plantings of tulips and daffodils, mascari and hyacinths were at their peak.
The second time was in May, just before I headed over to London for the Chelsea Flower Show, and I was fearful that there would be nothing to see at the Keukenhof. How wrong could I be. There were still huge drifts of tulips and many quiet areas with daffodils and fritillaria in bloom.
Comparing my two visits, I thought that the garden looks equally fabulous in April and May. I would recommend early to mid April if you want to see the iconic blocks of flower colour but you will find the garden just as beautiful in May when, by the way, all the trees are fully leafed out and looking their best.