Wow, there it was right in the _config file... Sorry for the trouble of even making a response. But thank you for responding so rapidly.
Just make sure you put the code in your /mysite/_config.php rather than altering the event_calendar/_config.php. Just makes it easier to install updates. You won't have to make the same modification every time.
I just found minor bug in format_character_placeholders and format_character_replacements arrays in CalendarUtil.php.
Took me a while to find where the problem. I was trying to get days without leading zeroes and used in _gonfig.php
following placeholders: "sDayNumShort" and "eDayNumShort". Outcome was days with leading zeroes.
Solution was obvious when I looked at those two arrays. In "format_character_replacements"-array replacements for "sDayNumShort" and "sDayNumFull" were swapped (in wrong order) and same goes also with "eDayNumShort" and "eDayNumFull."
I hope this helps somebody.