"Mieli wczoraj dobry dzień."

Translation: Yesterday they had a good day.

[jmwozniak]

... a nice day

[Neil395069]

They had a good day yesterday flows better in English

[Ryan329177]

I thought the same thing!

[sharkman.ru]

The hints on 'mieli' are: 'they had' and 'they not have'. How does it work exactly?

[Jellei]

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.