Hi,
Provided your system is set to use the German locale you can display an abbreviated day name using the
strptime() %a formatting directive. How you use the %a directive depends on what you want to achieve.
If you have a tag in a template, say $month.dewpoint.mintime and you want it to display just the abbreviated day name then something like $month.dewpoint.mintime.format("%a") will display (just) the abbreviated day name for the time the month minimum dewpoint occurred. I suspect though that the example you have given may in fact come from a template that uses a format that is picked up from the [StdReport] [[Defaults]] [[[Units]]] [[[[TimeFormats]]]] stanza or perhaps a similar stanza in a skin config file (skin.conf). You can certainly change the default setting in the [[[[TimeFormats]]]] stanza but be aware this will likely change the format used throughout a number of templates and skins. So it may not be what you want.
It might help if you tell us which template this is from or show us what template code you are using.
Gary