Can one insert an empty page? about cdcalendar

[liantze]

I don't think it's necessary to update the .cls. You can do something like this in the .tex file:

...
\end{monthCalendar}

\clearpage
\ThisCenterWallPaper{1}{image-file-name} 
% look into wallpaper package documentation for more commands:
% https://texdoc.org/pkg/wallpaper
\clearpage

% for the next month
\illustration{...}
\begin{monthCalendar}{2023}{02}
...

Or, if you had wanted a "cover page", you can use \coverImage and \makecover: look at the lines just after \begin{document} on examples how to set these. Currently the \makecover\clearpage is commented out so no cover is generated. You can un-comment them to see the example cover page.