This can't be done with CSS - to do this you need to have a cutom layer or theme.
You need to change the lang_posted_by_date_and_time function, which is:
function lang_posted_by_date_and_time(EntryLite e, bool showposter, bool showdate, bool showtime) : string {
var string posted = "";
if ($showposter) {
var string posterstr = (defined $e.poster ? ""+$e.poster : """$*text_poster_anonymous""");
if (not $showtime and not $showdate) {
return $posted;
}
}
if ($showdate and $showtime) {
if ($e.depth > 0) { # If $e is a comment
return $posted + $e->time_display("med", "");
} else {
return $posted + $e->time_display($*lang_fmt_date_med + " at %%h%%:%%min%% %%A%%M", "none");
}
}
if ($showdate and not $showtime) {
return $posted + $e->time_display("med", "none");
}
if (not $showdate and $showtime) {
return $e->time_display("none", "%%h%%:%%min%% %%A%%M");
}
return "";
}
In this, change these two lines:
return $posted + $e->time_display($*lang_fmt_date_med + " at %%h%%:%%min%% %%A%%M", "none");
return $posted + $e->time_display("med", "none");
, replacing $*lang_fmt_date_med with $*lang_fmt_date_med_day in the first and "med" with "med_day" in the latter.
This will give you the abbreviated day of the week at the start of the date and time. If you want the full day name, use $*lang_fmt_date_long_day and "long_day" respectively.
You can see all the date and time format options
here.