Are you using Xperia with Facebook app?
You can see if this app is enabled in auto sync in settings. If yes, then, click on it and there you will see Calendar option under Application control setting, uncheck it and it will remove all the calendar related stuff from the phone.
I also uncheck the Calendar sync option within the Facebook app setting which you access from inside the app.
Hope this help you. If it doesn't work try logging out from the FB app, clearing its cache and login again and make sure you remove this check before everything sync, otherwise you will have same issue.