I don't think there is a way to change normal messaging outside of using the tool's metadata.
I think there may be an idea to add metadata inside the toolbox, but I don't remember where it is.
However. It sounds like you have an excel file and then it checks for a sheet inside that's named like "January" or something. Why not just make that parameter a Table parameter? There isn't a default filter for it, but you can use updateMessages() to validate like
if '.xlsx' not in parameters[0].valueAsText:
parameters[0].setErrorMessage("Must be an Excel sheet.")
This way the user knows exactly which sheet they're using, and you don't have to guess about it.