Gmail has a feature that allows you to schedule when an email will be sent. This can be helpful for a variety of reasons, such as when you need to send a reminder in advance or when you want to send a "thank-you" note after the fact.
Here's how to schedule an email inside Gmail:
Open the message you'd like to schedule.
Click the down arrow next to Reply and click Schedule Message in the dropdown menu.
Select the day and time you'd like your message to be sent.
Click Add Scheduled Message at the bottom of the window and your message will be sent at the time you set.