May or early June weekdays are probably the best (basically any time the park closes at 8pm). However, May also has the most unpredictable weather of any month of the season here besides maybe October.
Of course, if you can afford at least one day of Fastlane Plus into your trip then it really wouldn't matter when you come, but I would still avoid July and August Saturdays. Even with a FL+ you may see 45+ minute waits for some of the more popular attractions like Maverick. Also remember if you are a Platinum Pass holder you can purchase a FL+ for the FL price in park only.
You get a discount on fast lane for being a season past holder?
I’m trying to find there fast lane prices for the season , it’s not posted yet.