The Sidebar is the area of the window on the left side that displays journals and entries. To show the contents of a journal, click on the arrow next to the journal. Clicking on any item in the Sidebar will select it and show the contents in the content area of the window. For example, clicking on a text entry will show the text for editing. Clicking on a journal will show some information about the journal itself. You can also select more than one item to see information about those items and perform actions from the menu bar or toolbar on all of them at once. If you have the Entries List open, you can see any entries you have selected, including any entries in journals.
At the bottom of the Sidebar is a calendar that shows one or more months. Each day that has an entry on it will be highlighted and clicking on that day will select the entry. If there are more than one entry per day, clicking and holding on the day will show a menu and let you pick which entry to view. Clicking on a day that doesn't have any entries in it already (and therefore isn't highlighted) will create an entry for that day and select it. One month is shown by default but more months can be shown by dragging the top of the calendar upward. To hide the calendar, drag the top of the calendar all the way to the bottom of the Sidebar or use the "Mini Calendar" menu item in the View menu.