With iOS 7.1, Apple has updated the Calendar.app with a new split view as you can see in the screenshot above. Luckily, jailbreakers can benefit of the new UI without having to update to the new iOS and loosing the ability to jailbreak.
Ryan Petrich has released ‘Gregorian’ on his own repo a free port of the iOS 7.1 Calendar.app. Gregorian will install as a separate app on your Home Screen instead of updating the existing app with the new UI.
To get Gregorian, add the following repo in Cydia: http://rpetri.ch/repo