countdown
Numbers that are counted backwards in fixed units from an arbitrary starting number to mark the time remaining before an event occurs.
On Danbooru, this usually refers to a series of images an artist or a group of artists draw with a number indicating how many days remain until a new character, event, product or series' release date or an old one's anniversary or birthday. They're usually indicated by "あとX日", meaning "X days left".
These illustrations should always be parented or pooled.
It's the opposite of event follow-up, which follows the progress of an event.
Often posted by official accounts with commissions from a variety of popular artists as a way of promotion.