Day One Plus and Premium users can create more than one journal. This is a great way to organize your data into different categories.
Plus (Grandfathered): 10 journals
Premium: Unlimited journals
You can also access these preferences by clicking Day One > Preferences (in the file menu) > Journals.
Version 2.9.2 or earlier
Click the + icon at the bottom left of the journals pane.
Select "New Journal"