Once you launch a campaign, especially to a large user group, ZetaHub takes a while to compute and prepare your email for sending. Once it has been computed and prepared, it is scheduled and sent.
You can find the campaign status at the top right of Activated or Scheduled campaigns.
You can also schedule campaigns to go out at a certain date and time when you build campaigns. These campaigns will be computed, prepared, scheduled and sent at the time you have specified.