Graduation season is a whirlwind, and backyard parties live or die by the planning you do while it is still cold outside. If you want a taco cart, you need to lock that vendor down by early spring so you are not left scrambling for leftovers.
A party for this size usually runs about $1500. By spreading that cost out, you keep your cash flow steady while you handle the rest of the graduation logistics.
The 0% intro APR window is not free money; it is just a way to pace your payments. If you still have a balance once the 15 months are up, interest will start to hit whatever is left.