Well, today I am excited to get to tell you that yes, there will be!
The demo will be released in September, before the full game is released. More details about what will be available in the demo, as well as the exact release date are coming soon, so stay tuned!"
As the release date is September 21st, I would venture a guess that the demo will be available sometime in the first week of September. Until then, you can sate your Civ thirst with some impressions.