Filter Entries by Custom date
You need to query using strings and your condition should be written as follow:
{% set catYear = craft.app.request.segment(-1) %}

{% set firstDayOfYear = catYear ~ &quot;-01-01&quot; %}
{% set lastDayOfYear = catYear ~ &quot;-12-31&quot; %}


{% set params = {
    section : 'termine',
    datumTermin : ['and', '&gt;=' ~ firstDayOfYear, '&lt;= ' ~ lastDayOfYear]
} %}

{% set beitrage = craft.entries(params).all %}

See documentation for reference :)