E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
On Wednesday, Thursday, and Friday of Passion Week, the Parisians go in procession to Longchamps, near the Bois de Boulogne. This procession is made by private carriages and hired cabs, and is formed by all the smartly-dressed men and women who wish to display the spring fashions. The origin of the custom is this: There was once a famous nunnery at Longchamps, noted for its singing. In Passion Week all who could went to hear these religious women sing the Ténèbres; the custom grew into a fashion, and though the house no longer exists, the procession is as fashionable as ever.