Thought I'd chime in since I have been experiencing a similar issue as well for about 2-3 weeks now. I have none of these issues when using my iPhone5S or my Gen 1 iPad Mini. My work around has been just doing all creations of new Calendar events from my i-devices for the time being. However...
On my MacBook Air (10.9.5) when I try to create a new event using either the "+" in the upper left, or by double clicking in the Calendar application there are issues. When creating a new event, I can enter whatever information in the Title, select any sub calendar like work , home, etc..., set the start/end times and date, and set alerts. However, when I hit "return" to save it, the new event loses all the text fields I have entered and reverts back to a 1 hour event titled "New Event" and also loses all configured, alerts, etc... When I double click on it and open it back up, I can re-enter all the information a second time and when I save it the second time there are no problems. On a rare occasion, when I use the "+" in the upper left to create an event, it will retain the color of the sub-calendar I selected and sometimes retain the title, but reverts to a 1 hour event with no alerts or any other information I might have added like notes, drive times, etc...
I checked Apple Discussions and there are several threads going on about this. Apple has suggested everything from reinstalling one's OS, upgrading to Yosemite, deleting plist files, etc... None of those have resolved the issue. Reinstalled OS's experience the issue, some Yosemite uses have the issue as well as Mavericks users, and rebuilt Calendars have the issue as well. Group consensus seems to be that it worked fine for years and years, and Apple must have changed something in the Cloud or on their backend somewhere. No one posting on the issue has mentioned doing anything preceding the issue which might have caused it like an upgrade or fussing with settings.