Thank you for the provided screenshot.
Changing the locale to pt-PT should switch to Portuguese with day names abbreviated as the following example demonstrates:
The Calendar resolves the short name representation of the day names from the localization service, and it's likely that they have been changed somewhere throughout your application. Please check the following demo for details.
To fix this, restore the original names (e.g. don't overwrite them).
If that's not the issue, I'd have to ask you to send a StackBlitz example or a zipped project demonstrating the issue, as I'm not sure how else to reproduce this. Thank you in advance.