To accomplish this on a blog setup, I ran:

mkdir ${SRCDIR}/templates
cp /usr/share/ikiwiki/templates/calendar* ${SRCDIR}/templates/
sed -i 's/^\(\[\[!calendar\)/\1 week_start_day="1"/' ${SRCDIR}/templates/calendar* ${SRCDIR}/sidebar.mdwn

where ${SRCDIR} was the source directory for the blog. This overrides the standard templates with ones that have the week_start_day="1" option added. If the upstream templates change, one has to manually update the locally overriding ones.

I personally managed to forget about having to change also the initially auto-generated sidebar.mdwn page, which led to some brain activity for me.