how to exclude several single dates from a diary block

[Rainer Thiel]

I use Org-Mode to schedule most everything, including my lectures
which typically are recurring events.  I have learnt that I can
exclude a certain range of days or weeks where no lectures take place.
For this, I use:

* TODO 12:15--13:45 Lecture: Aristotle
<%%(unless (diary-block 12 20 2021 12 31 2021) (and (= 3
(calendar-day-of-week date)) (diary-block 10 18 2021 02 11 2022)))>

which means that the lecture on Aristotle is scheduled for every
Wednesday between October 18th, 2021 and February 11th, 2022 – except
for the time between December 20th, 2021 and December 31st, 2021.

What I need to do for this year is to exclude single dates such as
April 26th, 2022 and June 21st, 2022.  Can someone please help me how
to achieve this?

Many thanks in advance,

Rainer
-- 
Prof. Dr. Rainer Thiel
Institut für Altertumswissenschaften
07737 Jena, Germany (EU)
r.thiel@uni-jena.de