make a custom macro and import it.
make a file called const.jinja inside custom_templates
Inside the file…
{% set weekdays = ['Montag', 'Dienstag', 'Mittwoch', 'Donnerstag', 'Freitag', 'Samstag', 'Sonntag'] %}
Then in your template
{% from 'const.jinja' import weekdays %}
{{ weekdays[(now().weekday() + 1) % 7] }}
Or you could try out my easy-time library for jinja that does this already. If your language isn’t supported, I can help you add it.