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<MyCustomView>(parameters))
    {
        EventDateTime = DateTime.Today;
    }
}

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

This makes the method applicable to multiple view models.

One thing that might help is switching to a "fail-fast" approach, where you test error conditions first and handle them immediately if one is detected. Execution in the method only continues if there are no errors, which means one less else block is required. That would give you this:

if (parameters == null)
{
    EventDateTime = DateTime.Today;
}
else if (!parameters.ContainsKey(PARAM_NAVIGATEDFROM))
{
    EventDateTime = DateTime.Today;
}
else if (parameters[PARAM_NAVIGATEDFROM] != typeof(MyCustomViewModel))
{
    EventDateTime = DateTime.Today;
}

Then, since the logic inside of all the blocks is identical, you could combine them into a single expression with a logical OR operator:
(This works because the operator has short-circuiting semantics.)

if ((parameters == null)                           ||
    (!parameters.ContainsKey(PARAM_NAVIGATEDFROM)) ||
    (parameters[PARAM_NAVIGATEDFROM] != typeof(MyCustomViewModel)))
{
    EventDateTime = DateTime.Today;
}

Alternatively, you could flip the logic around, testing to see whether the navigation is not coming from MyCustomView.

Instead of seperate if statements you could combine them into one:

public void OnNavigatedTo(NavigationParameters parameters)
{
    if (parameters == null ||
        !parameters.ContainsKey(PARAM_NAVIGATEDFROM) ||
        parameters[PARAM_NAVIGATEDFROM] != typeof(MyCustomViewModel))
    {
        EventDateTime = DateTime.Today;
    }
}

You could go further and move the check to a function which will make it a bit easier to see what you are checking for at first glance. Plus if you ever need to make the same check again you can re-use the function.

Personally I would invert also the conditions so that the function checks that it is from MyCustomView rather than if it isn't.

public void OnNavigatedTo(NavigationParameters parameters)
{
    if (!IsNavigationFromMyCustomView(parameters))
    {
        EventDateTime = DateTime.Today;
    }
}

private bool IsNavigationFromMyCustomView(NavigationParameters parameters) 
{
    return parameters != null && 
        parameters.ContainsKey(PARAM_NAVIGATEDFROM) && 
        parameters[PARAM_NAVIGATEDFROM] == typeof(MyCustomViewModel);
}