Check your google calendar settings on your computer on google.com. I had a problem like this once and found out that I had a "subscription" to Facebook birthdays. Just like how you can get a subscription to have the weather show up in your calendar.
If you open up your calendar app, not the widget. Top left you will see all calendars and a drop down arrow. Press the drop down and un check facebook calendar.
Sent from my International HTC One X using Android Central Forums.