I translated 'Mother's Day' as 'fête des Mères' which Kwiziq dinged as a mistake.
The 2013 Compact Oxford Hachette French Dictionary uses the translation 'fête des Mères' (p 719).
I read that there can be much variety in how such events are spelled.
Could it simply be that the spelling l used is not yet known by AI Kwiziq?
Thank you