The last week of April, some seven million bulbs are at their peak in the famous Kukenhof Gardens, about 45 minutes by train from Amsterdam. It has to be seen to be believed - acre upon acre of manicured grounds, trees, ponds (complete with swans), and everywhere beds of flowers, mostly tulips, but other bulbs as well. Rhododendron and Azaleas well.
Peak spring bloom only lasts for a couple of weeks, and the end of April is the most reliable time.
There is a summer flower season, as well, but it could not possibly compare.
The attached photos, taken on an overcast day, do not begin to do the place justice.