Almabtrieb

Did you already contact the local tourist office?
https://www.alpenwelt-karwendel.de/touristinformation-mittenwald

Likely they can help you best.

Yes I did but no info on mittenwald only other towns

Here is a link that takes you to the Almabtriebe website. The default is in German but there is a drop down option that will translate it to English.

https://www.almabtriebe.de/almabtrieb-und-almfest-in-mittenwald/

ETA: Here is a link to the Facebook page. I doubt that they know the date for next year, although this year it was the 3rd Sunday in Sept so chances are you are correct about that. However, you can always message them or keep checking back to FB to see if it shows up. https://www.facebook.com/almabtriebe

I wonder if each town knows already - so far in advance? I recall researching this in late spring / early summer for my September trip. I did not choose Mittenwald, but Mutters outside Innsbruck. That was a few years back.

Thank you for all your knowledge
I will check it out