Navigating to a view
I think Bethan has the best approach, but you could take it one step further and use a generic Type:

public void OnNavigatedTo(NavigationParameters parameters)
{
    if (!IsNavigationFrom&lt;MyCustomView&gt;(parameters))
    {
        EventDateTime = DateTime.Today;
    }
}

private bool IsNavigationFrom&lt;T&gt;(NavigationParameters parameters)
{
    return parameters != null
        &amp;&amp; parameters.ContainsKey(PARAM_NAVIGATEDFROM)
        &amp;&amp; parameters[PARAM_NAVIGATEDFROM] == typeof(T);
}


This makes the method applicable to multiple view models.