So I hope this is helpful...after upgrading to Mavericks from Snow Leopard, I am able to use the iCloud syncing between my iPhone and Mac. However, the vast majority of my iCal events on my iPhone (and what used to also be on my mac in SL) would not show up on iCloud or my new desktop iCal. If I created a new event on my iPhone, mac, or iCloud, it would sync fine in iCal. So what gives? I tried disengaging and re-engaging iCloud connections on my mac and iPhone, to no avail. I called Apple(non)Support and dealt with people who had to put me on hold to "do some research," just to come back and tell me they couldn't figure it out either.
What I knew was that the issue was on my iPhone. So, I just randomly when in to an event in my iPhone cal, clicked on the specific color-coded calendar that was associated with this event, switched it to another and back again, and voila! The event now appeared (almost immediately) on my desktop mac. So, pain in the *ss, but I went through my iPhone calendar and repeated this procedure for each event. What a tedious task!
Oh, and btw, don't switch between color-coded calendars too quickly or the iPhone will accidently make a second event.
What an annoying bug!