Article: With financial support from Sanders and Hubbard, Bell hired Thomas Watson as his assistant,[N 15] and the two of them experimented with acoustic telegraphy. On June 2, 1875, Watson accidentally plucked one of the reeds and Bell, at the receiving end of the wire, heard the overtones of the reed; overtones that would be necessary for transmitting speech. That demonstrated to Bell that only one reed or armature was necessary, not multiple reeds. This led to the "gallows" sound-powered telephone, which could transmit indistinct, voice-like sounds, but not clear speech.

Question: What month and day did Watson and Bell have a breakthrough?
June 2