Saw this last year and liked it so much I added something very similar to it to Infumap (https://github.com/infumap/infumap). You can drag items of arbitrary type onto dates. When more than one item is associated with a date, a numbered button appears; clicking it lets you cycle through them. Items can be pages or links to pages, which when clicked show the page as a popup. Calendar pages in the parent page display as a list of all items scheduled for the next seven days.