I think it is the same as the one on Stake. It is a part of the VIP club promotion where players get some bonuses such as weekly and monthly bonus. Bonus which is given based on user's account level and wagering activity during the last 7 and 30 days. I'm not active at Primedice but I'm active at Stake. Stake usually send the monthly bonus in the middle of the month but it can be early or a bit late, so there is no fixed date about the release of the bonus. Since Primedice and Stake are owned by the same team, so I do believe the monthly bonus is the same or at least similar in terms of the calculation as well as the time of releasement.