As you can guess, its in case a relative of the family dies, so they can appear to be in mourning akin to the national mood.
Her.ie has reported that there are also some other precautions taken with the royals with regards to food.
They write that Buckingham Palace never stock shellfish, due to the risk from food poisoning, in an attempt to minimise the chances of sickness and therefore withdrawal from royal and diplomatic duties.
The article continues:
"The Queen is rumoured to be very strict with these rules, however, many say Prince Charles has been a bit of a rebel and repeatedly enjoyed the fishy food."
The Daily Mail reported on this back in 2014, when top chefs that make up the Clubs des Chefs met at Hakkasan in London and discussed the eating habits of heads of state.
Rosaleen McBride said that the Queen does not eat shellfish and that Philip likes a lager with lunch, having spoken to the Queen's head chef, Mark Flanagan.
However, when President Obama visited the UK in 2011, crayfish was a starter on the Buckingham Palace menu - so we're not sure how much this information is to be trusted.
Meanwhile the Clubs des Chefs also revealed then-Presidents Barack Obama disliked beetroot and Francois Hollande detested artichokes.
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