Open the Messages app
Tap the compose button 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
Open Shortcuts
Tap Automation
Tap Create Personal Automation
Choose Time of Day
Set the date and time
Tap Next
Add the Send Message action
Enter the recipient and message
Turn off Ask Before Running if desired
Tap Done