Most time you send the email right away when you are done with it. However, you can schedule the sending time when you work with Gmail, please see below for details:
Step 1: To compose an email first by clicking the "Compose" button on the top left corner;
Step 2: Finish the "To" box and the "Subject" box, then finish the email;
Step 3: Click the triangle after the "Send" button and select "Schedule send";
Step 4: Select a time in the next window or click "Pick date & time";
Step 5: Pick up a date and type the time in the new window, and click the "Schedule send".