The original release date and time was July 16, 12:00PM CEST time. But there seems to be issues on steams end.
Currently the game is still in the review process. I sent the game in for review on July 3, which is almost 2 weeks ago. Usually it takes between 3 and 5 days for steam to do the review process and with all my other games, that is how long it took. But Keep on Mining! is taking longer.
I've sent in some help requests regarding this, so that might help.
I will release it as soon as the game has been review and accepted.
Sorry about this!