Botto is saying April is good month, because not only do you get to race kermesses (large fields of 100-200 most of the time), but all the pro classics action is in full swing from late March to end of April.
I'd prefer to go in Jul/Aug. The weather is warm. The fields are reasonable sized (30-80pp). Daylight until 10-11pm. And cafe season is in full swing.
May/June are bad for allergies, so be careful there. If you do get allergies, check in at the apotheek (pharmacy) and get some over the counter stuff. They have some great medicines that our American FDA makes us get prescriptions for.