From a serialization on the Vitaphone Varieties blog. (The studios in the late 20s would release serializations to the film mags based on the latest movie they were pushing. This one is in connection with the 1929 WB musical, Show of Shows - scroll down the blog page for background.)
(Leo has come to Hollywood desperate to get work in movies. His friend Harry promises to get him a job, even a small acting role, although Leo’s no actor, and arranges a place for him to stay.)
On the sidewalk again, Harry prepared to take leave of his friend to keep an appointment. Eliciting from Leo the tidings that the latter had no place to sleep, and no baggage, he gives him a key to his bungalow, mentioning an address on Gower Street, just off Hollywood Boulevard.
“Move in until your first ghost walks”, he invited.
This has the air of an established usage and Google Book Search throws up an interesting parallel from Berlin Alexanderplatz.
The higher-ups oughta know we’re on the job all right, and then maybe one of those chaps wants to get an increase in salary, his wife needs a fur coat, and that’s why they nab people and pick out a Friday, when the ghost walks.
The phrase seems to mean then ‘when you get your paycheck’. I can’t find anything in OED; has anyone alse seen this usage?