This is a known issue with iPhones. To fix this issue, please un-sync your calendar on your phone for about a minute to fix it.

Use the following procedure:

1. From the Home screen go to Settings > Mail > Accounts.

2. Select your Mass account.

3. Set the options to sync both mail and calendar to Off.

4. Return to the Home screen and go to the Calendar application. Verify that the calendar is empty.

5. Return to the Home screen and go back to Settings > Mail > Accounts.

6. Select your Mass account.

7. Set the options to sync both mail and calendar data to On.

8. Go back to the Home screen and go to the Calendar application. The calendar will re-populate with data from the server.