It might very well be 2 summer events... I can see the first 3 (Poseidon Boreas and Ignis) releasing for 1 month before the next set comes in (Metatron Gabriel Zeruel).
I've taken a look at Ryu-oh's weapon in specific, which is fundamentally a candy apple, essentially a motif of Japan's summer festivals (as far as I know, I'm no expert). So possible conclusion: likely a summer festival event. Edit: Which I searched through the DMM twitter, it is indeed a summer festival event.
As for which one? ... Beats me, I haven't immersed myself deep into Japanese culture at all. The best date guess I have is the "Awa Odori Matsuri" (translated Awa Dance Festival) during 12-15th August (the 3 yukata girls release at 23rd), but that seems unlikely to be the case for a couple of reasons. The more likely explanation is that this is more a generic summer festival event that isn't based off anything other than your typical anime summer festival. Edit: The DMM tweet doesn't specify what event it is modelled after, so I'm sticking to my second conclusion.
The tweet(s) are here btw, I simply did a tweet search range including the event dates for the 3 yukata girls.
https://twitter.com/search?l=&q=from...09-01&src=typd