This can be done in menu Main/School:
For each day you can write longname and shortname.
If you leave the field empty then the default names will be used. Default names are Monday,Tuesday(big suprise), in case of timetable for more than 7 days the default names are Day1,Day2...
You can also manualy switch to Day1, Day2 this way:
This article was viewed 146797 times.97886 / 146797