The displayed format is two letters only. But what if you want it to be displayed in three letters?
Here a small cosmetic surgery (i.e. code changes).
So, what does it take to do it successfully?
dayCell.appendChild(document.createTextNode(dtf.ShortestDayNames[(i + firstDayOfWeek) % 7]));
Searching the library code, I found that this array is defined in MicrosoftAjax.debug.js. as:
And fortunately, another array was defined with three letters names:
So all what it takes is to replace ShortestDayNames with AbbreviatedDayNames in both CalendarBehavior.debug.js and CalendarBehavior.js:
dayCell.appendChild(document.createTextNode(dtf.AbbreviatedDayNames[(i + firstDayOfWeek) % 7]));
It's great to have the source between your hands :)