Alright, it dropped down. At morning when I woke up and went to work it was -20 indeed. During midday while working I received a photo from our security cam (due to birds flying near the zone of the camera) and it shows also the temperature in photos. I saw it was still -20. Actually at morning, temperature often will cool more for couple of hours because of sunrise. I can't explain the reason for it.
But, oh what a beautiful shiny day. Few minutes ago I went to check a well that leads to a gutter and if the electric pump works ok beneath in the drainage. And then, at a same time I heard a "strix aluco", an owl, singing/shouting up in a tree. Wow! What an exciting moment.