Here's the breakdown:
* Main Phase 1: This is when you can summon monsters, set spells and traps, and activate certain card effects.
* Battle Phase: This is the only phase where you can declare attacks. You can attack directly (attacking your opponent's Life Points directly) if they have no monsters on the field. Otherwise, you must attack a monster on the field.
* Main Phase 2: This phase comes after the Battle Phase. You can activate certain card effects and perform actions like setting monsters. However, you cannot attack in this phase.
* End Phase: This phase is when you check for certain effects and end your turn.
So, to answer your question directly, you can attack during the Battle Phase only.