Do most of the important scenes in Othello occur at night or in the daytime? What is the effect on the atmosphere of the play?
Quite a lot of the play takes place at night. The play begins at night, with Iago and Roderigo (Act 1, Scene 1) waking up Brabantio to tell him that his daughter has escaped with Othello - and, of course Othello and Desdemona's wedding has already taken place in the middle of the night.
The meeting with the Duke of Venice, Brabantio and Othello (and everyone else) at night, where it is agreed that everyone will go to Cyprus takes place later that same night.
This night-time setting also adds to the feel of Venice as somewhere claustrophobic, uncomfortable, and really quite unpleasant. And that darkness is carried across into Cyprus, in attitude if not in setting. Several of the Cyprus scenes do seem to take place during the day, particularly the persuasion of Othello by Iago: adding, I think, to the casualness of things. It's so off-hand, just another day, and just another comment: 'Look to your wife'. And look at what happens.
Three key scenes do happen at night though. Firstly, the "drinking scene", with the drinking songs and the intoxication and eventual dismissal of Cassio. This interrupts Othello and Desdemona's wedding night - and Othello has to take Montano to dress his wound. Secondly, the scene in which Roderigo is killed by Iago in the dark, and Cassio is wounded. What both of these scenes have in common is Iago, as the puppet-master, running around in the dark, armed, and orchestrating the violence. It's a persuasive image of Iago.
The final scene - of course - which happens at night is the last one, and that dreadful build up to it, as Desdemona sings her "willow" song, and the wind blows outside. It's dark when Desdemona is strangled - she's killed at night. The darkness of Iago (who, of course, is *white* and not *black*: Shakespeare ironising the Venetian's racism) - spreads.
Hope it helps!