This is Read an Ebook Week at Smashwords -- a site-wide sales promotion. Authors can choose to enroll in the promotion and offer coupon codes of 25% off, 50% off, 75% off, and 100% off (free). I noticed that participation was a bit low last year. I have no idea what the participation rate will be this year, but I have already found a book that I had put on my wishlist for free, down from its regular price of $3.99.
The easiest way to check for titles in the promotion is to go to the main page, look for the Read an Ebook Week box on the left side, and click on the Browse Books on Sale button. Books can be added to the promotion throughout the week, so check back if you don't find anything right away.
https://www.smashwords.com/