Fifty is a real moment. The people, the room, the toast — that is what you'll remember, not the spreadsheet. So the job here is to give yourself a runway for the venue and the vendor deposits, then quietly spread the rest across a year so the party doesn't punch your checking account in the face.
Book the venue four to five months out. That's the hard deadline. Everything else — food, drink, music, the cake, the questionable photo booth — can flex around it. Lock the date and the room first, then price the rest against what you've already committed.
The 15 months is a deadline, not a gift. If $300 a month turns out to be too much, or the balance isn't gone by the time the intro window ends, you'll owe interest on whatever's still sitting there at the standard variable APR. Treat it like a timer: pay it off before month 15, or it stops being a deal.