I believe the next diamond special could be for World Sexual Health Day on 4th September.
Here is the text from last year:
While it may be the best thing on the planet to have sex in all imaginable positions and variations, it's always important to pay attention to proper safety.
As we celebrate World Sexual Health Day, from 03/09/2018 to 05/09/2018, we want to help raise awareness about safe sex and encourage you to spread the message!
Secure two exclusive items from the Sex Shop and show everyone that having fun being safe always go hand in hand!
During the special, you can also benefit from triple experience points from missions and pick up cool "Safer Sex" package at the Diamond shop - because you'll find not just sparkly diamonds in there, but also a 14-day skill and mission booster.
Happy World Sexual Health Day!