A backyard engagement party is all about the mood, not the overhead. You want to celebrate without starting your next chapter in debt, so focus on locking in your vendors early to avoid last-minute price hikes.
Booking on a weekday often gets you a better rate, as many caterers and rental companies have more flexibility when they aren't tied up with weekend weddings. Once you have your quotes, you can map out a payment plan that fits your monthly cash flow.
Remember that the 18-month window is a deadline, not a gift. If you have a balance left when the intro window on purchases ends, you will start paying interest on the remaining amount, which will cost you more in the long run.