... a nice day
They had a good day yesterday flows better in English
I thought the same thing!
The hints on 'mieli' are: 'they had' and 'they not have'. How does it work exactly?
I have no idea what the author had in mind when writing "they [not] have". Deleted it.
"Mieli" can be translated as "they had/they have had/they had had". For masculine personal plural.