To use a late day, instead of tagging your repository with the "hwN-final" git tag before the deadline, you tag it later, within 24 hours after the deadline. Generally, once you have tagged an assignment, you should not change your mind and re-tag a different commit. However, we will allow tags to be changed if you discover problems that require fixing. If you decide to do this, you must immeditely remove the old tag from your repo while you work on changes and re-apply the tag to the later commit when you are done.
As an an example, suppose that you have an assignment that is due on Thursday at 11:00pm, but you need extra time. You just don't tag your repository, and instead you can turn in that assignment (that is, add the git tag) by Friday at 11:00pm.