For an uninterruptible task that runs once per year for 24 hours, which Amazon EC2 pricing model is the most cost-efficient?

On-Demand Instances are the most cost-efficient for a task that runs only once per year for 24 hours. You pay for compute capacity by the hour or second with no long-term commitments, making it ideal for short, infrequent workloads. Reserved Instances offer significant discounts (up to 75%) compared to On-Demand, but require a 1-year or 3-year commitment. For a task running only 24 hours annually, the yearly commitment would be far more expensive than simply using On-Demand for that single day. Spot Instances are the most cost-effective for fault-tolerant workloads, but they can be interrupted with two minutes' notice, which is unacceptable for an "uninterruptible task." Dedicated Instances provide physical isolation for regulatory compliance but are significantly more expensive and not necessary for this use case.