Here are the message pages.
Some may be {{Null}}
Note that Type messages will be used if they exist, whereas Main messages will be used if there is no message parameter. This is because it is always possible to create a default type message, and it is never possible to create a default (non-null) main message.
Type messages have the parameters month name , year and cat available to them.
Main messages have month name and year.
Messages are called from Template:Monthly clean-up category/core