Open the Messages app
Start a new message or open an existing conversation
Type your message
Tap the plus button or app drawer next to the text field
Select Send Later if available
Choose the date and time
Tap Send
If Send Later is not available, use the Shortcuts app to create an automation
Open the Shortcuts app
Go to Automation
Tap Create Personal Automation
Choose Time of Day
Set the date and time
Add the action Send Message
Select the recipient and message
Turn off Ask Before Running if prompted
Save the automation