Temporary fix
Edit the following file on this path:
/wp-content/plugins/wunderground/templates/snippets/title.html
FROM
{% if showdata.daynames is not empty %}
{% if forecast.title is not empty %}
<{{tag}} class="wu-day-title">{{ forecast.title }}</{{tag}}>
{# Sometimes PWS stations don't have the title available #}
{% elseif summary.date.weekday is not empty %}
<{{tag}} class="wu-day-title">{{ summary.date.weekday }}</{{tag}}>
{% else %}
<{{tag}} class="wu-day-title">{{ strings.currently }}</{{tag}}>
{% endif %}
{% endif %}
TO
{% if showdata.daynames is not empty %}
{% if summary.date.weekday is not empty %}
<{{tag}} class="wu-day-title">{{ summary.date.weekday }}</{{tag}}>
{% elseif forecast.title is not empty %}
<{{tag}} class="wu-day-title">{{ forecast.title }}</{{tag}}>
{# Sometimes PWS stations don't have the title available #}
{% else %}
<{{tag}} class="wu-day-title">{{ strings.currently }}</{{tag}}>
{% endif %}
{% endif %}
THEN wait cca 30-60 mins for Wunderground cache refresh, and finally voilà since change just ‘names of days’ in a title…