It's permanent. Your just for a week question? nope it isn't.
You'll expect that a normal b-day event is just a new class, a new event (with shop) and a new update from the underground lab.
But not an enormous update of changing the hard to get reagents and items to ACs, good thing I did a bank glitch and store everything. Luckily, my bank is now less with the Nulgath items.