Update 8:15 AM EDT: Yogg-Saron's artwork has appeared on Hearthstone's Facebook page for some countries. Looks like we'll be seeing Yogg sometime this week!!
Whispers of the Old Gods Releases April 26th
The Naxxramas bundle in the Battle.net Shop has received a warning message before you get to purchase it. The message states that on April 26th you can only use the cards in Wild format. Since Standard and Wild will be launching with the expansion, it's safe to assume that Tuesday, April 26th is the release of Whispers of the Old Gods.
It should also be noted that there is still enough streamers to provide us with 2 weeks of voting card reveal content, so there's no way we'd even see the expansion before the 18th. An extra week for additional site reveals and the inevitable huge dump of cards right before launch is perfect.
Via /u/hythl0day on reddit
Last Week's Card Reveals
We saw 13 cards revealed in the past week, down from 14 the week before.
Learn More About Whispers of the Old Gods
Want to see more cards that are in Whispers of the Old Gods, or learn more about the set? Head on over to our guide to Whispers of the Old Gods to see the 45 currently revealed cards! Be sure to also check out Blizzard's Official Old Gods Page.
There's also a weekly card vote going on over on the official Hearthstone site! Go vote for which card you'd like to see revealed, new cards are revealed daily.