Focus on the joy of the day rather than the logistics. Getting the venue locked in two months out ensures you actually have a place to celebrate.
Spreading the costs over several months keeps your cash flow steady while you handle the smaller details like invitations and favors closer to the date.
The 18-month window is a deadline, not free money. If you have a balance left over after the intro period ends, you will start paying interest on what you still owe.