Question

I know how to navigate from one pivot item to another, within the pivot page, but what if the navigation is initiated from another page entirely?

UPDATE:

Here is the solution I used, based on the answer from Matt Lacey:

From button click event on MainPage:

private void button_Click(object sender, RoutedEventArgs e)
        {
            string parameter = "myPivotItem1";
            NavigationService.Navigate(new Uri(string.Format("/MyPivotPage.xaml?parameter={0}", parameter), UriKind.Relative)); 
        }

Overrode OnNavigatedTo for the pivot page, and extracted the querystring:

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
        {
            base.OnNavigatedTo(e);
            string newparameter = this.NavigationContext.QueryString["parameter"];
            if (newparameter.Equals("myPivotItem1"))
            {
                myPivotControl.SelectedItem = myPivotItem1;
            }
            else if (newparameter.Equals("myPivotItem2"))
            {
                myPivotControl.SelectedItem = myPivotItem2;
            }
        }

Answer 1

You can set the SelectedIndex to the index of the item you want.

Be careful of the delayed loading of item though.

Update:
On the first page, pass an indicator of which pivotItem to display in the querystring** of the page you are navigating to.
In the OnNavigatedTo event on the page, containing the pivot, create a handler for the Loaded event of the Pivot and use that to set the SelectedIndex.

** Other ways of doing this are available.