I would add a new readonly property called CountryName to your EventPage that's a string representing the country name (lazy load the country name as it will only be accessed when the EventPage is indexed by Find). Makes it real simple to search and facet on the country when searching events. 

Then I'd add a ContentUpdating event handler for your County page, use Find to look for all events in that country and update the related event pages.

Given I guess the countries are not renamed too often performance shouldn't be a big issue?

David

I asked a simular question a while ago and use the answer I got a lot and it works great. Read more here:

http://world.episerver.com/forum/developer-forum/EPiServer-Search/Thread-Container/2015/5/best-pratice-for-including-information-that-does-not-exist-on-content/

In summary it says that you should create a extension method for the pagetype and a function in it to pick up county name, then in a initialization module add something like this:

SearchClient.Instance.Conventions
    .ForInstancesOf<EventPage>()
    .IncludeField(x => x.CountyName());

This makes that Find will add a new property named CountyName to the object and it will only access it when indexing.

Later you can do terms filtering och things like that on it

Thank you guys! Extension method works like a charm :)

I need help with update method.

Init module:

private void ContentEvents_PublishedContent(object sender, EPiServer.ContentEventArgs e)
{
    var countyPage = e.Content as CountyPage;
    if (countyPage != null)
    {
        var eventPages = SearchClient.Instance.Search<EventPage>()
                                        .Filter(x => x.County.Match(countyPage.PageLink))
                                        .Take(1000)
                                        // return any field
                                        .Select(x => x.PageLink)
                                        .GetResult();

        foreach (var searchHit in eventPages.Hits)
        {
            SearchClient.Instance.Update<EventPage>(searchHit.Id)
                        .Field(x => x.CountyName(), countyPage.Name)
                        .Execute();
        }
    }
}

1. Can I update EventPage(s) in one go, or do I have to update them one by one?

2. The above code throws the following exception:

An exception of type 'EPiServer.Find.ServiceException' occurred in EPiServer.Find.dll but was not handled in user code

Additional information: The remote server returned an error: (403) Forbidden.

Your key is not authorized to access (POST) '/INDEX_NAME/Models_Pages_EventPage/_700f7047-ce29-456d-9403-af5d6205584e_no/_update'

Thanks

If you are using Update, I think you need to do them one by one. 
It would be better to just index a batch of EventPages

SearchClient.Instance.Index(eventPages);