A good portable projector turns a backyard into a theater for years. You want something with at least 1080p resolution and enough brightness to handle the sun going down, not just pitch black. Buy a solid one now so you aren't replacing a blurry, dim unit in twelve months.
New models usually drop in the spring, which makes fall the best time to buy. Retailers clear out last year's stock during holiday sales. The older version often has the same lamp life and ports as the new one but costs a few hundred dollars less.
The 0% interest rate is a tool for time, not a discount on the price. If you still have a balance after the 18 months, the standard interest rate kicks in on whatever is left, making your projector much more expensive.