I had a Home calendar and a Work calendar. All of the Home calendar events disappeared after loading Mountain Lion. I was able to get the events back though.
I went into Time Machine and went to the backup just before I loaded Mountain Lion. I went to my user name/Library/Calendars. Under Calendars I could see the calendars I had set up, but they were named things like 2230360F-C7E1-4A86........ I didn't know which one was which, so I clicked on each of them. Once you open it there will be a folder titled "Events". For each Event folder I clicked on it and then hit the space bar. A pop-up window came up and I looked for the folder that had been updated most recently (I use my Home calendar a lot, so I knew it would have the latest update). Once I found that folder I 2 finger tapped it and selected Restore "Events" to... A Finder window popped up and asked where to restore the folder too. At this point I could navigate to my Calendar, but it was greyed out and I could not select it. So, I created a new folder on the desktop and restored the Events to there.
I exited Time Machine and opened the desktop folder. I chose Select All and then Open With - Calendar. A pop-up window appeared telling me that a new event was available and what calendar to put it in. I hit Allow and it started loading. The only problem is that a new pop-up window will appear for each event - I had about a hundred on my desktop that I had to close out individually, but all my events were loaded back in though.
I've just installed Mountain Lion on my computer, upgraded settings to iCloud so I could sync my new iPhone (which thankfully I haven't done yet) and half my Calendar events have disappeared. I haven't got a time machine back up as nothing and nobody suggested that I needed to do a back up before installing Mountain Lion.
This is so irritating. The Apple Store couldn't transfer my contacts from my old phone either because they didn't have the up-to-date machinery. I'm going to have to input contacts manually. Am I going to have to do the same with my Mac Calendar? People seem to be saying that even if I update on my Mac it won't stay there because iCloud is the Master. What does that even mean?
Just discovered the same problem with calendars.
When I installed Mountain Lion after it was released I did not set-up the iCloud account.
I only work an iMac and have a 2010 8Gb iPod touch software ver 4.2.1 for music. I do not bother to sync to it.
I discovered that the only calendar available was Birthdays under the Subscriptions heading in the left pane and nothing in my troubleshooting capabilities could find how to enable my old calendar data.
I created a new calendar called Home to try to populate but again couldn't figure out how to get my data into it before giving in and enabling iCloud. Articles in Macworld et al didn't help too much.
So then I followed this thread which helped persuade me that I needed to set-up my iCloud account properly.
Having done that I now have my calendar data with a new calendar named Work created in the left pane under iCloud. So now there are 2 calendars under iCloud - Home and Work with some historical data in both (selected by tick box) and under Subscriptions - Birthdays.
Whilst setting-up iCloud in Sys Prefs I was asked to chose an iCloud email address to use with Mail and Notes and that I needed to enable UPNP in my Router system settings to allow Notes to function.
Looks as if this was the answer for me.
Hope that helps others.