i have only just returned from months travelling and decided to trade in my stars that ive had sitting there for ages and about to expire. I heard about this about a year ago and was waiting for it.
Anyway, after a few days of constantly trying I have found out some of the times that points become available and have been able to get 3 x 300 and 1 x 100 cards.
You can only get 1 card a day so it might take some of you a while to spend those points.
There are over 200 x 300 point cards made available at midnight every night, thats the ones I have been going for, I also found that the 100 point cards are avaialable around 10pm so you can grab one at 10pm and another after midnight if you missed out the previous day.
As of yet I have no idea when the 500 or 1000 point cards are out and I will probably have spent all my stars before I figure it out.
Good Luck.