Thanks for the answer and let me at first excuse my poor English, I'm not a native speaker :-(
I'll try to explain my question once more and my be that it is not related to the "Post Types Order" plug-in?!
Normally when not using the "Post Types Order" plug-in, the last post, which is topmost in the order also causes an entry in the calendar widget. The day of the last post is underlined or highlighted. I don't know how this works?!
What I wish to have when using the "Post Types Order" plug-in is a similar functionality.
After my re-ordering of posts via the plug-in, I also would like to have an underlining or highlighting of the topmost post!
Any idea to manage this?