If you're ready for the post to go live right away, you can select the 'Launch Now' option. The post will immediately be listed in the Newest feed and users can begin interacting with it.
Additionally, you can schedule the post to go live at a future date/time if you're not ready for the post to go live at this time. The site operates on PST so the time and date you select are in PST. You can find more information about the scheduling option here.
While a post is scheduled, you can continue to edit it before it goes live. You'll see your scheduled post listed on your profile.
You can find additional information here: