diary-mark-entries-hook is a variable defined in `diary-lib.el'.
Its value is

  • This variable may be risky if used as a file-local variable.

List of functions called after marking diary entries in the calendar.
You might wish to add `diary-mark-included-diary-files', in which case
you will probably also want to add `diary-include-other-diary-files' to

This hook runs after `diary-nongregorian-marking-hook'. These two hooks
differ only if you are using included diary files. In that case,
`diary-nongregorian-marking-hook' runs for each file, whereas
`diary-mark-entries-hook' only runs once, for the main diary file.

You can customize this variable.