Press your device's < Home > button, then press < Menu >.
Select Applications, then Manage applications.
Select Calendar Storage.
Note: If you don't see 'Calendar Storage,' press < Menu > again, then select Filter > All. On some devices, you won't see 'Calendar Storage.' It is fine to ignore this step if it doesn't apply.
Select Clear data. Answer OK when prompted.
Restart your device. Press and hold the < Power > or < End call > button, then select Power off. Press the < Power > or < End call > button to power on.