A good backyard movie setup should give you years of weekend entertainment, not fall apart after two uses. You want a bright projector and a stable screen that won't flop over in a light breeze, so buying decent gear once beats replacing cheap stuff next year.
Late summer clearance is the best time to buy as retailers clear space for fall inventory. Last year's projector models usually offer almost the exact same picture quality for a much better price, making them the smarter pick.
Remember that the intro window is a fixed deadline, not free money. Pay off the full balance within the 18 months, or you will face interest charges on whatever amount remains.