Does the Steam client automatically download pre-ordered games after their release?

I've pre-ordered Grand Theft Auto V on Steam. When I pre-ordered, Value cheekily charged the full amount to my credit card, so I'm all paid up. Just waiting for it to drop.

If I leave my Windows PC on, with Steam logged in and energy saving disabled, on the evening of Monday, 13th April, 2015, will my Steam client automatically start the download after midnight—or do I have to stay up and click a button? By all accounts, the download is huge, and I'd like to start it as early as possible so it has the best chance of being ready by the time I put my three-year-old to bed on Tuesday evening.

Being old and grumpy, I'm not very keen on staying up past midnight on a work night if I don't have to.

Steam does not auto-download preordered games. It doesn't auto download any games. It only downloads what you manually select for install, and since you can't install preordered games yet, you are out of luck. Except if it does offer preloading as mentioned by Orc JMR in the comments.

you actually can, by installing the steam app on your phone, you can remotely install the game when you're away. but you must leave your computer running with steam on