diary-comment-start is a variable defined in `diary-lib.el'.
Its value is

String marking the start of a comment in the diary, or nil.
Nil means there are no comments. The diary does not display
parts of entries that are inside comments. You can use comments
for whatever you like, e.g. for meta-data that packages such as
`appt.el' can use. Comments may not span multiple lines, and there
can be only one comment on any line.
See also `diary-comment-end'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.