A pig roast is about the smell of wood smoke, the music, and having the whole street over for an afternoon. It is the kind of event that turns neighbors into friends, but coordinating a pitmaster and sourcing quality meat takes a bit of lead time.
You want to lock in your vendors two months out to avoid a scramble. By spreading the $1500 cost over a longer period, you keep your cash flow steady while you focus on the guest list and the sides.
That 18-month window is a deadline, not a gift. If you have any balance left after the 18 months, the interest kicks in, which will quickly make that pig roast much more expensive than you planned.