I stayed in Haarlem in April 2018 and toured Keukenhof on the 24th of April, took the bus from Haarlem and arrived fairly early, before 10 am I think. We toured, ate lunch there and left by 1 or 2 pm. Wonderful time there! You will love the gorgeous colors and designs of the displays.
On King's Day, I wandered Haarlem throughout the day and enjoyed the festivities, talked to people, listened to the bands, took pictures. it was a hoot, everyone was dressed in orange and just had wholesome fun. An interesting thing is, people bring household items they want to sell in a giant flea market! Very fun though, I just talked to people and looked over their wares. A great people watching day.
If you go to Keukenhof in the morning of King's Day, the 27th, you will be able to enjoy Haarlem in the afternoon. Lots of local color, orange that is!