The login client, the small window that appears where you push "play" to get into the game, usually has all limited time offers, sales and other Crown Store related and event related info. So take a look there before actually entering the game.
Other than that, major Crown Store releases such as limited time goods are usually announced via sosial media such as Facebook and Twitter, and there's a Crown Store showcase being linked on the ESO webpage and here on the forums every month.
But for the limited sales such as this one, your best bet would be the login client.