When sending a bulk mailout, you have the option to either send using Predictive Delivery or setting it at a Scheduled time.
You will have this option during the last step of setting up your mailout:
Predictive Delivery: Allows you to send each mailout at the same time of day your customer last made a puchase (we see this increase open rates). This will send over a 24 hour period,
Same Time Delivery: You can schedule your mailout to go out all at once, and you can select to send right away, or at a set later date and time.