I did until I uninstalled a calendar widget I'd added, can't remember the name of it now, but if you've installed an app that accesses the calendar, try taking it out and seeing if that sorts it. If you're syncing the calendar data to another source, try going to the home screen, hitting menu, selecting settings, going to app manager, and scrolling to the calendar storage stuff and hitting "clear data" then power off then back on again. It should then re-sync and may sort it out. I've done that once or twice with my calendar while I was trying out various apps, but haven't needed to since my first week with the phone.