I would say, visiting not during a heat wave and not during biting gnat season (April through Mid-June) would be the best times. -- Our first visit was in mid-August when temperatures were between 90-100 degrees and it was HOT. Since there is no electricity in the camping area, meaning no A/C unless you start your generator. Otherwise, you may want to stay when it's below 85 degrees. Evenings did cool down nicely. Early to mid-August is also when they celebrate Spider Fest (Google it) because they are very impressive and prevalent at that time. The spiders did not "bug" us, but some people are not fans. Speaking of bugs, the biting gnats (no-seeums) are out in the spring (April through Mid-June), so we planned our 2nd visit here for late June and did not experience any. Late June was also rich with spotting baby bison, baby pronghorn, and baby swallows around the back of the visitors center.

April, b4 the heat and the bugs