The ban must be announced within 28 days of the the card becoming legal in the format in paper OR the card becoming legal in the format online (i.e. ignoring any pre-release events).

Pre-bans don't count, the card has to spend some time being playable in the format before it's banned.

The card must be banned from one of these formats: Standard, Alchemy, Explorer/Pioneer, Historic, Timeless, Modern, Legacy, Vintage, Pauper, Commander, Brawl, Standard Brawl. Bans that hit a subset of one of these formats (such as a card only being banned in Best of 1 Arena Standard) still count.