Awh man... I thought it was every Friday. I was planning on doing some heavy Merit grinding for rank 3. Now i'm stuck at rank 2. 
the way to know when it is going to tick over is: roll a 20 sided dice, whatever it lands on, share that by 3, add 1, then throw the result away and take a guess. Seems to be what FD expect us to do. Fuming over this, I lose out on 465 merits because FD cant even supply us with the required info OR actually you know, make the cycle last a week!