See my above statement.
Apple IS NOT UPDATING ANY OLDER OS X VERSIONS ANY LONGER!!!!
No more UI updates or tweaks. NONE.
Apple is on to OS X 10.11 El Capitán, now.
There is NOT going to be any system updates to older versions of OS X.
Apple may still support security updates for a little while longer to older versions of OS X and still allow updates to iTunes under older versions of OS X, but THAT IS IT!
Live with the way OS X 10.9 Mavericks looks and feels OR upgrade to the free El Capitán OR dial back to and pay to install OS X 10.8 Mountain Lion.
I think the Calendar app in OS X 10.8 Mountain Lion is the same as it is in Mavericks.
And the Calendar has not improved that much in later versions of OS X.
If anything both Yosemite and El Capitán have, what I feel, is this gawawful flat 2D icon look, like iOS is, now, and I think it just looks gawdy and unappealing as a UI interface.
The Calendar app in the later OS X versions, may be just as unappealing.
Apple changed the look of many apps AFTER OS X 10.6.8 Snow Leopard.
So, if you do not like the look of what Apple did with certain apps, then you will need to search for alternatives that more suit your needs.
Its the way it is, now, and hasn't really changed since.
Good Luck!