Stuff Your Kindle Day" is a promotional event offering thousands of free eBooks across various genres, including romance, mystery, and science fiction. These eBooks, normally priced up to $14.99, are available for free download on specific days throughout the year from Amazon and other retailers. Participants can keep the downloaded books permanently. The event is designed to help readers expand their digital libraries without cost. Additional details, including how to participate and access the free eBooks, are available on the event's page.