The expansion, titled "Conquer the Seas" will go live today, after a few brief delays. A developer post on November 18th set the expansion for "late next week", but a week or so of delay is almost certainly worth it for a bit of extra polish on the expansion. This is the largest patch that Adventurine has ever released, and more ambitious projects are certainly more subject to bumps along the way.
Today's developer post says "We're getting ready with the expansion as we speak," and notes that the EU servers will be down for the patch and could possibly be down for 24 hours. The NA1 server will remain up during that time, then experience brief downtime for its own patch. Developer Tasos emphasizes that EU players will be reimbursed for their downtime.