Scheduled Reserved Instances
aws.amazon.com/blogs/aws/new-scheduled-reserved-instances
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Our new Scheduled Reserved Instances are a great fit for use cases of this type (and many more). They allow you to reserve capacity on a recurring basis with a daily, weekly, or monthly schedule over the course of a one-year term. After you complete your purchase, the instances are available to launch during the time windows that you specified.