Turbot or Halibut?

Alistair & Margaret Miller (YC's Canadian uncle & aunt) have been visiting Tynemouth and staying in four star luxury (possibly) at number 10.  The weather was rather cold and grey, but they found time for a bracing walk along the pier and a stroll to North Shields fish quay.  On Thursday night, we repaired to The Grand Hotel with Andrew & Diane for a very enjoyable dinner.  Fish was on the menu, and the following conversation ensued:
Diane - is this turbot?
Andrew - no it's halibut.
Diane - oh, I thought it was Thursday.
Andrew - so am I, let's have a drink.