The answer?
Is virtually every year. In June. When I go on holidays. At least once (maybe three years ago) I badgered them into letting me change my flight to the day before the planned strike (Tuesday to a Monday). This was the Saturday before. Think the strike was called off on a Sunday but that wasn't any help as it was getting to close to my travel date and I had to travel.
I'm near to done with them. I've two flights in June and July and if either fall through then I'll be using Lufthansa to connect in Europe and US carriers for North America. It's a total joke at this stage.