June gets booked up fast, so the real trick to a backyard graduation party is getting your catering locked down at least two months in advance. Once you secure the date with the taco spot, the rest of the backyard setup and grocery runs fall neatly into place around that deadline.
A catered taco bowl bar for a crowd usually runs right around $900. Breaking that up over time makes it much easier to handle while you are buying invites, drinks, and decorations.
That 18-month intro window is a hard deadline, not free money, so make sure the balance is gone before time runs out or standard interest kicks in on what is left.