Abbreviations for days in Polish

I was watching the local polish language news program the other day, and during the weather forecast the days were abbreviated (I am pretty sure Czwartek was just Cz). I cannot seem to remember the abbreviations- can someone please help me?

There are no formal rules. Some of the used forms are:

poniedziałek – pon., pn.
wtorek – wt., wto.
środa – śr., śro.
czwartek – czw., cz.
piątek – pią., piąt., pt.
sobota – sob., so
niedziela – nie., niedz., ndz.

I glanced at those days and suddenly realised their ethymology - like a bolt from the blue sky!
Czwartek would be Fourther in English, while piątek - Fifther.
Amayzing - I have lived so long being ignorant about this origin. :):):).