Replacing text for all files in a directory

[Kemp Randy-W18971]

Since I don't write Python every day, here is a simple question.
I have a directory called test, containing files A, B, ..., Z, and I want to
replace all occurrences of the text "Have a good day" to "jump off a cliff."
This is a comic example, but I need to do something similar in real life.
How can I do this in Python, running on Unix?