I integrate the ebusd in HA via mqtt. I generate the needed discovery messages myself, but I read that ebusd can also generate the discovery messages.
As the write you’re doing is a single operation with multiple parameters in a single call you cannot use builtin discovered MQTT AFAIK. There may be a way tweaking mqtt-json.cfg file but it’s way out my abilities (btw I hate regexp).
What you can do is publish an already crafted MQTT message to the specific topic, in your case it should be
ebusd/dwh/holyday_date/set
and use a template as a payload that converts a datetime input in a format that ebusd understand.
I choose to use an offset over the current date and time as “start a custom setting from now and keep it for X hours”, I use a template like this
{% set newtime = now()+timedelta(hours=states('input_number.temp_override_hours')|int) %}
{{ '"' + (states('input_number.temperature_override_temp') | float)| string + ';' + (newtime.strftime('%H')) + ':' + (newtime.strftime('%M')) + ':' + (newtime.strftime('%S')) + ';' + (newtime.strftime('%d')) +'.' + (newtime.strftime('%m')) + '.' + (newtime.strftime('%Y') + ';257"')}}
where
input_number.temperature_override_temp is the new value of the CH/Cooling temp to be set
and
input_number.temp_override_hours is the number of hours for which the new setting must be maintained
you can always adapt the logic to a different behaviour.
Glad to know the constant is not mandatory, I’ll look into it as soon as the cooling season starts…
Thank you so much @ wrongisthenwright ! 
With your hint I was able now to automate this holiday setting depending on my calendar entries. 
It took me some time tonight to adjust it to my needs, but finally it´s working. 
Glad it worked, I’ll see what happens with mine in a few weeks…
I got two addresses mixed up in my previous post on the datapoints of the Ariston Nuos split. The correct addresses are: 1366 for the evaporation temperature and 136c for the suction temperature.
The HA card now looks like this:
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Which model should I get? Because this one (Daniel Kucera) keeps blocking my boiler every now and then
The eBus adapter shield C6 (Welcome to eBUS Adapter Shield - eBUS Adapter Shield C6) worked for me out of the box, without any change in settings or trimming needed.