Long weekends like May, July and August long weekend are very busy times for campgrounds. Many campgrounds require you to book for the entirety of the long weekend (such as May 15-18th). This is so that one site is not being used for just one day of the long weekend, or used by three different travelers. This helps keep the campsites clean, maintained and available for everyone to enjoy for the long weekends.
If no sites are showing up for your preferred dates, try changing the dates by a day or two so you can see what similar dates are available for such sites.
