Can I rename the days?

The default names for the days are - Monday, Tuesday, ... In case of timetable for more than 7 days the default names will be filled with "Day8, Day9, ..."

Note:
If your work weeks start with other days than "Monday", do not rename the days, but set the weekend at first:
See: How to change weekend to "Friday-Saturday". It will automatically rename all days in a week accordingly.

You can change the names of the days in main menu School - Rename days - select the day and click the Edit button:

For each day you can specify the long name and abbreviation

You can simply switch to numbered days - "Day1, Day2,..." names in this way:
We need Day 1, Day 2... rather then Monday - Friday