It never happened. John died at the start of S2, 2 seasons before Cas was introduced in S4. Although there have been 2 time travelling episodes (In The Beginning & The Song Remains The Same), Cas didn't meet John in either. In the first, he only appeared to speak to Dean when he was alone, and when Cas returned Dean to the present John was still dead au unconscious. In the second, Cas spent the entire time in the past recovering in the motel room.
