In Jekyll, by default, you cannot view a post with a future date. If you try to, you will see this message
Skipping: _posts/2020-09-07-post-name.md has a future date.
If you want to view it anyways, it's really simple.
View post with a future date in Jekyll.
All you have to do is add the --future flag.
So in your terminal, do bundle exec jekyll serve --future or jekyll serve --future depending on which one works for you.
You will now be able to see the future post.