For standard SMS messages, users have the option to send the message immediately, or schedule the message. If you click on schedule, there is a Send On field where you can set the date & time you want to send out your text message.
SMS Quiet Hours

To comply with government regulations and mobile carrier preferences for marketing communication, you cannot schedule an SMS message to deploy between 8pm and 8am in the recipient’s local time zone. Any messages which are queued that fall within these hours will be sent the following day.
Sending a Standard SMS
If you selected “Send Now”, click on the green button “Review & Send” to confirm your setup and initiate sending your text message. If you selected “Schedule”, click on the green button “Review & Schedule” to confirm your setup and schedule the deployment of your text message.