At the Schedule/Send Mailing screen, you will be able to send your email immediately or schedule it to be sent at a later time.
For large mailings, it is often wise to schedule the sending overnight when there is less demand on your server.
Send Immediately - Click the box to send the mailing.
Schedule Mailing - Choose the year, month, day, hour and minute you want the mailing sent.
Click <<Previous to return to the previous step, or Submit Mailing to send or Cancel.
Note that your mailing will not begin until the CiviMail cron php script is run. This script can be executed manually or set up as a cron job. Refer to the Setting up the Cron Job section in the CiviMail Installation Guide for details.