So when is that dentist appointment, Mr Outlook? 19th or 26th?
Top feature expected from a calendar: show me clearly what occurs when.
Of course you could argue "obvious, events are hanged just right under their date", which puts my dentist appointment on the 19th.
But consider the case where you want to add an event to an empty day. By selecting the white cell, are you 100% sure the upper label applies to it, rather that the lower one? Same problem with days full of events. It would help to make very clear which label comes with which cell, through a better visual association.