Is "nos dydd Iau" wrong? It wasn't accepted.
If you translate 'nos dydd iau' it will be 'Thursday day night' so no we drop the dydd
I may be wrong but translation from welsh to english wont actually make any sense for a lot of sayings
So dydd and nos relate to whether there is still daylight? Prynhawn dydd iau seems to be correct even if there is no "day" in afternoon
In the case of prynhawn dydd Iau the dydd can be dropped without losing the meaning.