Each day (or sometimes more than once per day), a full 'compose' of the tree is attempted. This will usually result in the creation of all or most of the usual install, live and disk images, installer trees and so forth. The composes are synced to thedirectory on the mirrors, and you can find the images there.
Each successful compose is tested by openQA and a mail summarizing the results is sent to the devel and test mailing lists, so you can check the openQA interface or the 'compose check report' emails to check whether that day's compose is installable. You may also use the nightly image finder tool maintained and hosted by a Fedora QA team member, which conveniently offers the last completed build for each image and the last that passed all tests, for openQA or Autocloud-tested images.
Using nightlies in the past was a fragile way to install $RELEASE, but with improved compose processes since Fedora 24 and automated testing since Fedora 23, their quality has improved substantially and this will often result in the best experience.