Hello, Elle!
I have heard of Quadragesima Sunday.
Could it be to do with Lent or Easter or similar?
It is to do with the Church, anyway.
No, I have not heard of "backstory", too.
Looks like it should be "back story", to me?
Andy Capp! That's a blast from days gone bye, surely!
Wonder if that was at the back of my mind?
Can't make any sense of it with the clue though.
The darts one, Elle.
The "forty" mentioned in the clue could be what is known as a "double top".
At the top of the board is "20".
A dart landing in a certain bed in the "20" segment scores double, giving forty, "double top".
Your cousin and you seem to well organised for chatting!