A solid set of wheels changes how your bike feels more than any other part. These should last you years and thousands of miles of chunky gravel and dirt. Buying a quality set once means you aren't replacing dented rims or fixing broken spokes every other season.
New bike tech usually drops in the spring. That makes autumn the best time to shop because retailers want to clear out inventory. You can often grab last year's top-tier carbon or alloy wheels for much less than the original price just because the stickers changed colors.
This window isn't free money forever. If you still have a balance left when the 15 months are up, the standard interest rate will apply to the remaining amount. Stick to the monthly plan to avoid extra costs.