For "he is in a bad mood" we have the choice between "on jest w złym humorze" and "on ma zły humor", is it thus possible to say "Mam dziś dobry humor" for this one? (Sorry for any mistakes)
Yes it is :)
Can we accept a different word order? Jestem w dobrym humorze dziś
It doesn't really sound natural. You put the new information at the end of the sentence, so in your sentence "today" is the new information. Which is rather a weird thing to say.
I wrote : Jestem w dobrym humorze dziś .
Why is it wrong?
It's not wrong per se, it's unusual. It's like a calque of the English word order. Usually we don't put adverbs at the end of the sentence.
Basically it's like saying "I am in a good mood TODAY".
why not humorem?
You are "in a mood" both in English and in Polish, so it needs Locative.
What about Jestem w dobrym humorze dziś ?
Sorry - should have read all the other posts!
What's wrong with putting 'today' at the END?
I'm in fact in a very bad mood today and really should not have been doing this. Seventh anniversary of my wife's passing after 47 years and a very lousy day.