There's a calendar that goes up until November of this year back on page 32 (post 316) of this thread but I was curious about the pattern myself like you. So I went ahead and mapped out all the updates and made a calendar that goes up through 2019, lol. It took me a bit to figure it out but I did it so if they keep going as they have then the pattern will remain 15, 15, 5. If you're interested or want to make up your own calendar I'll explain in detail below, if you're not interested just ignore the following....
It seems pretty simple but there's a catch that makes it a bit confusing: when the first fifteen day goal ends there's a day of rest then the second fifteen day goal begins. But when the second fifteen day goal ends, the five day goal starts that very same day. When the five day goal ends, a fifteen day goal starts later, continuing the pattern.
As an example, we'll use the current Bowling goal as the first 15 day goal: It started Tuesday Sept. 18th and finishes today, Oct. 2nd. We'll get a day of rest on Wednesday the 3rd then the new 15 day goal will start on Thursday the 4th run until Thursday the 18th but the 5 day goal starts that same day and will end Wednesday the 23rd. Finally, on the 23rd we'll get another 15 day goal to start the cycle all over again.