calendar-today-visible-hook is a variable defined in `calendar.el'.
Its value is

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

List of functions called whenever the current date is visible.
To mark today's date, add the function `calendar-mark-today'.
To replace the date with asterisks, add the function `calendar-star-date'.
See also `calendar-today-invisible-hook'.

In general, be careful about changing characters in the calendar buffer,
since it may cause the movement commands to fail.

You can customize this variable.