Unfortunately, the calendar app in Mavericks does not have the days separated with lines as some older calendar versions had. As far as I know, you can not go back to the older calendar - at least not the normal way. However, there are other calendar applications you can use with your Mac. Check the Mac App store by typing in "calendar" in the search box and then choose one that you like, some are free, others are not. There are too many for me to mention here.