Starting in version 7.5 for iOS and macOS, Day One supports a Trash for deleted entries. The Trash is located below the list of journals in the Journal Drawer/Sidebar.
Restore deleted entries
Open the journal drawer/sidebar and go to Trash near the bottom. Tap an entry and then tap Restore. The entry will go back to the journal in which it originally existed.
Swipe from right to left on an entry for additional options including Select which allows selecting multiple entries to restore simultaneously.
On macOS, select the entry and click the Restore button at the top.
Permanently delete entries
In the Trash, entries can be permanently deleted as well. Once permanently deleted, entries cannot be recovered. Similar to restoring entries, permanently deleting entries can be done in the Trash.