Yes its permanent. I got CuriousCanidae to take a pic for me because I'd bought all the BGs already, but it has NOT lapsed for even a day and its been over a year, and this screencap was taken 11 months after and Korea server doesnt even have summer shit up when it was taken. STOP ASKING IF ITS PERMANENT, YES IT IS.

This will be very OBVIOUS when you see it available during Christmas.

If it wasn't permanent they'd tell you when it will leave.