Above is an admission ticket to the "annual ceremony of washing the lions," held on April 1st. (From the collection of the British Museum.)
The first recorded instance of this prank was in 1698. And in 1848, the year the Fox girls' story begins, people were still falling for it. Here is Gustave Louis Maurice Strauss' detailed account of the prank:
"I think it was in the last days of March, 1848, that the proprietor of Chat, in conjunction with the editor and Pond, controved to perpetrate a vile hoax upon Her Majesty's lieges. These wretched conspirators had a great number of order-cards printed, admitting "bearer and friends" to the White Tower, on the 1st day of April, to witness, if they so listed, the famous grand annual ceremony of washing the lions. I am sorry to say that I was over-easily prevailed upon to join in the distribution of these favours among friends and acquaintances.
We went to Tower Hill in the morning of the 1st of April to watch the result. I must confess I, for one, was not prepared for the extraordinary credulity of the British Public. They flocked up in shoals to see the lions washed. The "warders" were almost at their wits’ end. They had the bits of pasteboard flourished in their faces, with angry gestures and angrier imprecations, by the indignant crowd of sight-seers and seekers. I verily believe there was a notion at one time of the day to send for reinforcements of the garrison, so threatening was the aspect of the B.P. raging at the gates of the old City fortress. In the midst of the turmoil some one spotted me to whom I had given an order of admission, and he would have set the whole mob upon me, but I most luckily succeeded in securing the friendly shelter of a cab, which I made drive off instanter from the field of action, knowing of old that discretion is, as a rule, the better part of valour. The final result to me was, that I had to skedaddle, and keep dark for a time, until the affair had blown over a little."