You are turning thirty, and a rooftop party is the right way to celebrate it. Getting the space locked in takes planning, and you want to start booking at least two months in advance to get the spot you actually want.
A night like this usually runs about $1500. Lead-time is your real leverage here, letting you lock down dates early and spread the cost out so you are not scrambling to pay for everything at once.
That intro window is a strict deadline, not free money, so make sure the balance is fully paid off before those 15 months run out or you will get hit with accumulated interest.