Messages that are waiting to be dispatched are kept under the Scheduled heading in the Gmail navigation pane on the left, and until a message has gone, you can open it up, edit it, change the scheduled time it needs to be sent, and delete it.
Future schedule suggestions will usually include the last date and time you picked, so you can easily get a batch of messages going at the same time.
Gmail will make some suggestions for you, like tomorrow morning, but if you choose the Pick date & time option at the bottom you can specify exactly when you'd like the email to go. Gmail is best at this, and if you're using Google's email client on the web, you can click the small arrow next to the Send button when composing a message, then Schedule send to pick a day and a time. As well as scheduling your text messages, you can also put a delay on your emails.
