First birthday parties are really for the parents, and that is completely fine. You want a fun backyard setup without spending a fortune or rushing at the last minute. Giving yourself a full month of lead-time lets you order bulk decorations online before high retail prices hit you at the local party store.
A realistic budget for a cozy backyard setup runs around $400. If you use the right tool, you can break that total into tiny monthly payments over time instead of taking the hit all at once.
A 0% intro APR is a deadline, not free money. If you do not pay off the $400 balance within the 15 months, the standard variable interest rate kicks in on whatever amount is left.