FileResult with MemoryStream gives empty result .. what's the problem?

Just a guess: Do you need to Seek back to the start of the stream before you return it?

fs.Seek(0, 0);

iCalendar iCal = new iCalendar();
foreach (CalendarItem item in _db.CalendarItems.Where(r => r.Start > DateTime.Now && r.Active == true && r.CalendarID == ID).ToList())
{
    Event evt = new Event();
    evt.Start = new iCalDateTime(item.Start);
    evt.End = new iCalDateTime(item.End);
    evt.Summary = "Some title";
    evt.IsAllDay = false;
    evt.Duration = (item.End - item.Start).Duration();
    iCal.Events.Add(evt);
}
// Create a serialization context and serializer factory. 
// These will be used to build the serializer for our object. 
ISerializationContext ctx = new SerializationContext();
ISerializerFactory factory = new DDay.iCal.Serialization.iCalendar.SerializerFactory();
// Get a serializer for our object
IStringSerializer serializer = factory.Build(iCal.GetType(), ctx) as IStringSerializer;
if (serializer == null) return Content("");
string output = serializer.SerializeToString(iCal);
var contentType = "text/calendar";
var bytes = Encoding.UTF8.GetBytes(output);
var result = new FileContentResult(bytes, contentType);
result.FileDownloadName = "FileName.ics";
return result;