Good question! And the answer is - it depends on your interpretation of the play.
It's become a theatrical convention, I think, since Peter Brook's hugely influential 1970 staging of the play to double up Theseus and Oberon (i.e. have them played by the same actor) and Hippolyta and Titania. This has the effect of implying that, as per the first scene, the whole of the action "in the forest" is the pre-wedding dream of either Theseus or Hippolyta: and, of course, as the two seem to leave the stage at the end of Act 1, Scene 1, on an impasse, when they "re-enter" as Oberon and Titania, they are having a huge argument.
However, as the grand lady of Shakespearean criticism, Professor Anne Barton, has pointed out "How could Theseus possibly dream Bottom?". That is - if it is a dream of Theseus or Hippolyta, how could the translation of Bottom (into a donkey) make any sense in it?
Who else in the play gives us a hint it might be their dream? Well, Bottom does:
I have had a dream, past the wit of man to say what dream it was.
And, in the same scene, so does Demetrius on behalf of the lovers:
Are you sure
That we are awake? It seems to me
That yet we sleep, we dream.
Perhaps Shakespeare didn't want there to be one single, definitive answer!