If you experience a calendar sync issue on the HTC EVO 4G, take the following steps:

1.  Remove the exchange account from the phone.

2.  Go to Settings > Applications > Manage Applications and click ALL at the top of the screen.

3.  Find Calendar Storage and tap the Delete Data button.

• Make sure any personal calendar data that is not in the Exchange account is backed up before clearing data as this will delete calendar entries displayed on the device from the local calendar.

4.  Reset the exchange account and sync on the phone.

http://community.sprint.com/baw/thread/33420?start=0&tstart=0