Here are your 2019 dates
https://disneyparks.disney.go.com/blog/2018/06/2019-disneyland-resort-grad-nite-dates-announced/
Grad Nite is when seniors from around CA, AZ- really any within 8 hours driving distance I'd say have the chance to celebrate their graduation with a school trip to Disneyland and a seniors only after hours party in DCA. They also have the chance to buy discounted access to DCA and Disneyland day of their party. This can lead to huge crowds of high school seniors in the parks, especially in the evening hours as everyone has arrived and they want to do all their DL favorites before hittin the party in CA Adventure. Whether your seniors are behaved or not is a crapshoot. Some people report perfect angels, others are having "F this" "F that" "*****!!!" and a million other things screamed out while waiting in lines and even on rides. There are also reports of ride photos being ruined (if someone shows a finger, flashes the camera that will get the photo removed/blacked out and you do not get another chance to ride).
Scared yet? Don't be

I've travelled during Grad Nites and the parks are totally doable with a bit of pre-planning:
1) park hopper tickets
2) do not enter Disneyland after dinner, unless you're in a gambling mood
3) do not enter Disneyland after dinner.
You can search the DIS for more info and you will see everyone has an opinion on Grad Nites and often they're polar opposites.
WOC has in the past only showed once on Grad Nites.
Personally, I like the week after Memorial because odds are you'll have longer hours and more-full entertainment schedule. BUT you
will have bigger crowds. Hotels are generally more expensive the farther into May you get. The past years
sometimes they run ticket specials for SoCal residents that run until Memorial(?).
The Dark Horse here is SWL opening. It's opening "summer 2019". This could mean Memorial, this could mean August, nobody knows. If you want to for sure avoid SWL stuff early-mid May is safest on that front.