Does anyone know how I should define a transit service that only occurs on public holidays, in GTFS? 

```
calendar.txt as;
'4,0,0,0,0,0,0,0,20161211,20171209'

calendar_dates.txt as;
'4,20171001,1'
'4,20171002,1'
'4,20171225,1'
'4,20171226,1'

```

This apparently does not seem to work... My dataset also contains normal regular services, and they work fine.