"...a journal application for Ubuntu that keeps one page per day that it is use..."So, in an another sense, it is actually a very simple Text editor, again a very simple one. You won't be getting a toolbar for colorizing or formatting your texts... but if you want to create and access your daily notes with ease then Daily Journal is pretty darn good!.
Like two days ago they released a new version which brings a very useful feature that it lacked for sometime. Which is to let the users to "navigate" their daily notes via a calender type GUI (as in the screenshot). Anyhow, if you use Ubuntu, then you can install it using a PPA channel.
For that, open your Terminal and enter the below command.
sudo add-apt-repository rickspencer3/ppa
sudo apt-get update
sudo apt-get install daily-journal