Busman's holiday
A busman's holiday is an idiom meaning to spend a vacation doing the same thing one would do in their normal profession. I first came across the reference in Busman’s Honeymoon (p 149):
“I'd be glad enough if you'd stay; you might give me a bit of help, seeing as you know the ropes, so to speak. Not but what it'll be a kind of busman's holiday for you.”
"That's what I was thinking," said Harriet. "A busman's honeymoon."
There was movie called "Busman's Holiday" in 1936. Sayers's book came out the following year, in 1937. Perhaps this was a common phrase at the time.
Busman's holiday
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