Hmm. You may want to share a few more details in your questions. It sounds like you're looking for something very specific about Miller's way of writing the play, and to be frank, I'm not sure I know what it is. I'll answer in general, and then you can tell me if I’m close.
In general, the term "point of attack" in playwriting refers to when the author/audience enters the story. It is pretty common for the point of attack be close to the final resolution in modern plays (a few days or even a few hours before things all get tied up). All the earlier history leading up to that climax has to be communicated somehow. In Miller's case, this is done mainly through flashbacks and through the bits of back story communicated in the dialogue.
In the heart of the play itself, the point of attack is from one night through the following day. In the requiem section, some time has passed and it is now Willie's funeral, but the core point of attack is the last day of his life.
It is that moment nearest the beginning of the play in which the major conflict to be resolved occurs; sometimes called the inciting moment. It is the moment in the story at which the writer decides to start the play's action. In the story, it was done through bringing us to the past and then bringing us back to the present or you could say flashback and some of the dialogues.