One of the great things about ebooks is you will NEVER be charged a late fee as AFTER it expires the file is no longer usable. But, yes you can renew the day it expires and up to 3 days before it expires.
Here are some more details:
Renewing digital titles works a little differently than renewing physical books from the library. When you renew a digital title, it doesn't extend your lending period. Instead, it lets you borrow the title again immediately after your current checkout expires (if there are no existing holds) or it places you on the wait list to borrow the title again as soon as possible (if there are existing holds).
You'll see the option to renew a title three days before it's set to expire. A Request again button will appear under the title on your Loans page.
For detailed instructions on how to renew an eBook or an eAudiobook, please visit the OverDrive help site.