When is end of billing cycle?
Hard to tell exactly ... On mine it looks like a new billing cycle starts on the 29th, but I've also had months where it switched as early as the 25th ... yours could be completely different.
Have a look at your statements and try to figure out, but I would suggest you go pay the 50% today or tomorrow and the balance on Monday, 30 May.
EDIT: ... and if you take someone along with a Student card, you might score an extra 5% ... every bit counts.