Unfortunately there's no set date or time, as it isn't really a thing that Plarium do.  I think that the last time they did it was part of an anniversery thing for them, so it might be tied into something similar in the future

Rose is correct in that there is currently no set date or time for more events. Really hoping to see one sooner than later. 

Agree with the other two. It seems to happen once a year at least, and the next one should be relatively soon. 

That said - last year's one was considerably less friendly than the year prior. From what I remember, last year they did a handful for specific ones, but only as tournament rewards.