Can I change the day name from Tuesday to Tu?

Yes, here are the steps:

  1. Open your project and go to the Main tab. Click Month Style to open the grid editor.

    Open the Month Style

  2. In the Template Editor tab, click Weekday Names.

    Open Weekday Names

  3. Erase the text field, then click Paste Special and pick the shorter name variant. The preview on the right shows you how the cell will look like. We went with the first variant:

    Pick a name variant

  4. To get the desired result, repeat the steps for Weekend Names:

    Repeat the steps for the weekend names