Juan 14:20

Jesus post-resurrection appearances would convince the Eleven of His deity. He described this condition as mutual abiding with the Father (cf. John 14:10-11). Moreover these appearances would also convince them of their union with Jesus. They would do so by confirming Jesus’ promises of their union with Him (John 14:13-14). Jesus expounded both abidings later (John 14:23-24; ch. 17).

Some interpreters take the day in view as referring to Pentecost. [Note: E.g., Tenney, "John," p. 147; and Blum, p. 324.] However because of the flow of the argument "that day" seems to refer to Easter rather than Pentecost.