I am planning a 3-day visit and trying to decide on dates. I am pretty flexible starting June 21 (about the earliest I can go) through Aug 14 (latest I could go). Looks like right at the start or end of my window are perhaps the best bets. After looking at a few crowd calendars (this one, undercover tourist, and isitpacked.com) I am thinking either 3 days starting either June 21 or 22 or 3 days starting Aug 11. The crowd calendar on this site has the June dates as (surprisingly) low crowds, but some other site I saw didn’t (8/10 on the undercover tourist one… which had low crowd on the Aug 11-13 dates whereas this one has them mid/low). It will be a first time trip for me and I will have little kids along (6 yrs and almost 4 yrs at the time of travel). I know that nobody can predict the future but thought some more experienced Disney-goers may have some insight. Thanks :)!
I would vote June–mainly because it will be brutally hot in August (now it might be in June but there is chance it will just be hot).
I wouldn’t base my vacation solely on crowd calendars (there’s too much variability and are likely to change between now and then).
I came here to say the same thing as @Dreamer. August has the potential to be brutally hot. The worst times I have ever been to Disneyland have been in August. Once it was a heat wave with daily temps over 100 and I swore I’d never go then again (how quickly I forget*). June ranges from pleasant to hot but the morning fog often keeps it from getting too bad.
*I will be going in late August so even I will roll the dice from time to time. It might be fine. 
As a resident of SoCal for 45 years, I can attest that August is hotter than June but remember it’s a dry heat so it’s much more bearable than Florida.
Be aware that these CLs are not definite until about 60 days out. (Really until about 45 - 30 days)
Right now they are just using historical data for 2018 / 2019. Also, sometimes that far in advance they are just using the historical data from that calendar date and not adjusted for the day of the week.
For example if your trip starts on a Monday, that day was probably a Sat. or Sun. in 2019. So you might see a CL10 because this year that “date” was a weekend. When they update the calendar in 2020 they’ll adjust for that date being on Monday this time.
I love TP but any CL # that is more than 60 days out I no longer trust. Wait and see… it’ll fluctuate. TP only updates about 60 days in advance and copies & pastes the previous year’s historical info into the farther away future dates.
My first DLR trip is Sept. 2020 - it’s one of the slowest times of the year due to school being in session. (I don’t have to take kids on this one - empty nest celebration!)