Just check randomly. I found the db devil today, and I've found it 3 times this past month. I only check randomly on Friday, Saturday, and Sunday. I check around 3-4 times on those days. You don't need to rely on some time guide bs. Just check randomly and hopefully you get lucky.
P.S- you hit the db devil 10times less than the other monsters
ex. I hit normal waterdevils 2.5k. and the db devil 250dmg
P.S.S- Although it attacks you with normal attack, it has range. It spawns up any of the four islands in which the normal ones thunder you.
That's from my experience and good luck to you