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

Open the Shortcuts app

Create a new personal automation

Choose Time of Day

Set the desired date and time

Add the action Send Message

Select the recipient and message

Turn off Ask Before Running if prompted

Save the automation