This message appeared on the Dominion Online Client. For those who haven’t logged in recently this is what the message says:
In early November, Dominion: Nocturne will be released. We’re happy to announce that during previews (the week of Oct 23-27) you will be able to play with the previewed cards on this platform. Look for the previews at all fine places where people talk about Dominion, such as dominionstrategy.com, boardgamegeek.com, and /r/dominion. The previews will go up each day at around 6 PM UTC.