During which part of flight is V1 speed critical?

V1 speed is critical during the takeoff phase of flight because it represents the maximum speed for aborting takeoff. When the aircraft reaches V1, the pilot must decide whether to continue the takeoff or to abort it in the event of an emergency. If an issue arises before reaching V1, the aircraft can safely stop on the remaining runway. However, once V1 is reached, the aircraft is committed to taking off; it is generally considered unsafe to attempt to stop, so pilots focus on safely continuing the takeoff and handling any potential emergencies in the air. This makes V1 a crucial speed for understanding potential takeoff performance and decision-making in flight operations.