I have found this instruction to install
dconf-editor, but I do not know the %letter formatting to switch "Mon Dec 18" to "Mon, 18 Dec", nor do I see how to use dconf Editor to accomplish this task.
You need to set
%a, %d %b in the "custom-time-format" entry.
%a: Weekday as locale’s abbreviated name. (Mon)
%d: Day of the month as a zero-padded decimal number. (18)
%b: Month as locale’s abbreviated name. (Dec)
You must also set