When free days were first introduced, they were a big thing. Some writers claimed to have built careers on them (based on follow-through sales to other titles, greater visibility, etc.
The problem is that free is only effective if it's used sparingly. What happened instead was that we ended up with a glut of free books. And not just on Amazon. The last time I checked, almost 20% of the books in the Smashwords store were permafree.
Also, Amazon changed the affiliate payment structure because so many sites were making affiliate bucks on the back of free sales. With that change, most sites that advertised free books, often for free or for very nominal fees, disappeared overnight.
In other words, the way I use free days is...I don't. Free is sufficiently commonplace to have become an only marginally effective strategy at best. Maybe if you have a long series and do free promo on the first book, that still works to some extent. But just the act of making a book free, even if it gets a lot of downloads, doesn't really accomplish much anymore.