By default, the Mac app creates a backup of your full journal every second day, keeping the 20 most recent. The settings for both frequency and retention can be changed in Preferences > Backup.
The default location for v1.7 and above is:
The default location for v1.5.5 and below is:
~/Library/Application Support/Day One/Backup
To restore from a backup:
First, open Day One preferences (Day One > Preferences) and select the "Sync" tab. Make a note of your current settings (i.e. whether you're using iCloud, Dropbox, or a custom location). If you are using a custom location, make a note of the path to the journal file.
Next, navigate to the backup folder.
The simplest way to navigate to the backup folder is from within the Day One app:
Open the Day One preferences (Day One > Preferences)
Click the "Backup" tab
Click "Show Backups Folder in Finder"
If you are unable to use the app to get to the backup folder, you can find it manually in Finder. However, the Library folder is hidden by default. To open it:
Open a Finder window
Click the "Go" menu.
Hold down the "Option" key on your keyboard
An entry for "Library" will appear in menu list. Click it.
Locate Containers/com.dayoneapp.dayone, and right click that file
Select 'Show Package Contents'
Proceed to this path:
Once you have found the backup folder you will see a series of backup files that look like this:
Journal Backup 2012-08-10 (1344661016).zip
Journal Backup 2012-08-13 (1344891774).zip
Journal Backup 2012-08-14 (1344980333).zip
Find the backup for the date you'd like to restore. If there are multiple backups on a single day, the most recent is the one with the larger number in parentheses.
Double-click the selected zip file to decompress it. One of the following will appear:
If "Journal_dayone" appeared, rename it (click it once, then press "Enter" to rename) to "Journal.dayone".
Double-click the "Journal.dayone" file. Day One will ask if you would like to use the selected journal file. Click the "Yes" button.
You are now using the backup journal, but are not finished yet. You need to merge the backup data into your original journal.
Open Day One preferences (Day One > Preferences). Click on the "Sync" tab. If you were using iCloud prior to the restore, click the "Use iCloud..." button and follow the instructions. If you were using Dropbox, click the "Use Dropbox..." button and follow the instructions. If you were using Day One Sync prior to the restore, click the "Use Day One Sync..." button and follow the instructions. If you were using a custom location click the "Custom Location..." button and select the journal file you were using previously.
Upon successful completion of the previous step you will have restored your backup to your original location. If you wish, you can return to the backup folder and delete the "Journal.dayone" file. The zip file backups should remain.