Walking into a room full of your favorite people while someone yells surprise is the kind of moment that makes a fortieth birthday unforgettable. Pulling that off without a massive headache means nailing the logistics early so you aren't scrambling at the last minute.
Timing is everything for an event like this. You need to lock down the venue at least two months in advance to get the date you want, leaving the rest of the planning to fall into place behind it.
Remember that the intro window is a hard deadline, not free money, and any remaining balance left when those 15 months end will start collecting standard interest.