If date is between conditional
This conditional should work:

{# Saison Trigger #}
{% set saison = now|date('m-d') %}

{# Check for each Season #}
{% if saison &gt; '11-22' or saison &lt;= '03-21' %}
    {% set currentSaison = 'winter' %}
{% elseif saison &gt; '03-21' and saison &lt;= '06-21' %}
    {% set currentSaison = 'fruehling' %}
{% elseif saison &gt; '06-21' and saison &lt;= '09-22' %}
    {% set currentSaison = 'sommer' %}
{% elseif saison &gt; '09-22' and saison &lt;= '11-22' %}
    {% set currentSaison = 'herbst' %}
{% endif %}


You're right, Twig does just compare the strings. But this is totally fine in this case. A string representing a date and formatted like so "01-13" (don't forget the leading zero) works nicely with the comparison operators.