A Notion page icon is usually an emoji or a fixed image. Animated icons are images generated for the current day: a calendar icon shows today's date, a countdown drops a day, a progress ring moves forward a little.
Once set as a page icon, it also shows in the sidebar and in links to the page. They're grouped by theme below, and you can change colors, text and other options.
Choose one from the animated icon library, click Create and sign in.
Set the date, text, colors and other options. The preview shows how it looks today.
Copy the image link, click the page icon in Notion, choose to add a custom image by link and paste it. It updates by itself every day.