Last couple have come out in late August or early September for the following year. Obviously Galaxy's Edge could affect that--if it does come out in September, they will almost certainly add an update early next year that will be available for free to anyone who already purchased that year's edition.
I was wondering about this also. I went to buy the 2020 edition and saw that it was still the 2019 being offered. A lot is changing with Galaxy's Edge and the Epcot construction so I'd like to have an edition that incorporates that info.