A Notion cover is usually a still image. Animated covers move and change with the time: the sky shifts from morning to night, a holiday counts down, the year progress moves forward each day.
Copy the image link, then in Notion click Change cover → Link. Backgrounds like the README banner and tech stack also work in a GitHub README.
Choose one from the animated background library, click Create and sign in.
Set the title, date, colors and other options.
Copy the image link, then in Notion click Change cover → Link and paste it. The cover changes with the time by itself.