Instructions for iOS and macOS below.
Go to Day One > Settings > Journals > tap the Journal for which you need to locate the ID > Advanced > Journal ID. Tap the journal ID to copy to the clipboard.
Go to Day One > Preferences > Journals > click the "i" icon to the right of the journal. Locate the journal ID at the bottom left of the journal information details.
Go to the Server Dashboard at https://dayone.app and sign in. The journal ID is listed below the journal name.