Attempting to set the 31st of December to a non-working day, seems to change the

WORKAROUND

1. Log in as a System Administrator
2. Enable en_US
3. Make en_US the primary language
4. Sign out
5. Log in as a System Administrator
6. Mark the desired dates as non-working
7. Save the changes
8. Disable en_US and make en_GB the primary (if this is the requirement from the customer)
9. Sign out
10. Sign in and make sure everything looks good. If not, contact Appian Support asap.

Thanks Eduardo, that works