Calendar: Highlight certain holidays

[Rolf Ade <rolf@pointsman.de>]

I use calendar and then its function holidays often enough, that I set
the variable calendar-holidays in my init.el.

It occurs with me, that I would be helpful, if the holiday list would be
colourful. The one added by me to calendar-holidays in another face than
the holidays coming out of the default. And then also different faces
for the holidays, I added (e.g. for distinction between public holidays
and memorial days (say Mothers Day)). 

I looked into it, but there is nothing obvious. The user is able to
control, what holidays appear in the buffer, that lists it, but every
entry in this buffer is equal to each other in how it is displayed. 

I was in the mood to accept some effort as a proof of concept, that you
can everything with emacs, I would have used a hook in
calendar-in-read-only-buffer, if there would be one.

I'm somewhat disapointed, that I'm not able to come up with a sane
concept, how that could be done. Do I miss something?