With the new permission schema in iOS6, app should acquire authentication before accessing calendar. So the code for calendar operation should be surrounded like this:

EKEventStore *eventStore = [[EKEventStore alloc] init];
EKEvent *event  = [EKEvent eventWithEventStore:eventStore];

[eventStore requestAccessToEntityType:EKEntityTypeEvent completion:^(BOOL granted, NSError *error) {
    if (granted) {
        // Calendar operation
    }
}];

As you can see, the code for calendar operation is written in a block.

After the operation completed, you may want to show customer an alert box to say the operation is done. But, showing a UIAlertView object in block will not perform expected action. The reason is because of the event loop. I am still studying it, so will update it later.

The correct way to show an alert box is using dispatch_async function to put the operation block to main queue, which looks like this:

[eventStore requestAccessToEntityType:EKEntityTypeEvent completion:^(BOOL granted, NSError *error) {
    if (granted) {
        // Calendar operation
        dispatch_async(dispatch_get_main_queue(), ^{
            UIAlertView *alert = [[UIAlertView alloc]
                                  initWithTitle:@"Event Added"
                                  message:nil
                                  delegate:nil
                                  cancelButtonTitle:@"OK"
                                  otherButtonTitles:nil];
            [alert show];
        }

    }
}];