So ...

If you are a Classic Editor fan, you need to check out the "Publish" meta box in the top right corner of the editor screen.

As you can see, the default option is "Publish immediately" ...

However, if you click the "Edit" link, a new scheduling option will be revealed allowing you to select the date and time when you want to publish your post:

Once you are happy with your choices, click the "OK" button and the traditional "Publish" button will change into "Schedule" ...

Click it, and Wordpress will publish your post on the date and time you entered.

Finally, let's see some closing thoughts ...