On the mobile app, the calendar under the "Schedule" tab in "My Posts" gives you the ability to see the big picture and organize all of your social media marketing efforts so you know what goes out when. Your posts are organized by the date they were sent to social media. On the web app, you'll find your calendar under the "My posts" tab by selecting "Schedule."
If you tap or click on a particular day, you can see any past posts that were sent to your connected social media accounts. You will also see when future posts are scheduled. A green dot shows posts that were sent on that day. A blue dot indicates a post on that day is scheduled, and a red dot indicates there was a problem sharing a post on that day.
For more information on viewing your past posts check out this tutorial.