Mark 13:32 says “But of that day or hour no one knows, not the angels in heaven, nor the Son, but the Father alone”. 

Why/how is it that neither Jesus nor the angels knew when He would return?

That the angels don't know should not surprise us. Angels, like us, are but finite, created beings (Colossians 1:16), and thus are not omniscient. Additionally, not only do they not know when Jesus is coming back, they also don't understand the gospel (1 Peter 1:3-12).

What is a bit surprising, at least at first glance, is how Jesus could not know the day or hour of His coming (again), especially considering that:

• He saw Nathanael under the fig tree (John 1:48),

• He knew men's unvoiced thoughts (Matthew 9:4, 12:25, Luke 6:8),

• He predicted His own death, burial and resurrection (Luke 24:7),

• He predicted Judas' betrayal (John 13:18-30),

• He predicted Peter's denial and repentance (Luke 22:31, 32), and

• He foretold the destruction of the temple in AD 70 (Luke 21:6).

But He didn't know when He was coming back. Weird, right?

Remember...Christ had two natures: human, and divine. As a man, Jesus was finite in all ways (Hebrews 4:15); as God, He was omniscient (John 16:30, 21:17). So how does this work out?

Jesus gives us some clues:

Therefore Jesus answered and was saying to them, “Truly, truly, I say to you, the Son can do nothing of Himself, unless it is something He sees the Father doing; for whatever the Father does, these things the Son also does in like manner. “For the Father loves the Son, and shows Him all things that He Himself is doing; and the Father will show Him greater works than these, so that you will marvel. “For just as the Father raises the dead and gives them life, even so the Son also gives life to whom He wishes. — John 5:19–21

“For I did not speak on My own initiative, but the Father Himself who sent Me has given Me a commandment as to what to say and what to speak. “I know that His commandment is eternal life; therefore the things I speak, I speak just as the Father has told Me.” — John 12:49, 50

Jesus, by His own confession, only did what He saw the Father doing, and He only said what the Father told Him to say.

Paul sums this up in his epistle to the church in Philippi:

Have this attitude in yourselves which was also in Christ Jesus, who, although He existed in the form of God, did not regard equality with God a thing to be grasped, but emptied Himself, taking the form of a bond-servant, and being made in the likeness of men. Being found in appearance as a man, He humbled Himself by becoming obedient to the point of death, even death on a cross. — Philippians 2:5–8

So, Christ, co-eternal and co-equal in essence with the Father (and with the Holy Spirit), set aside his divine prerogative and all the "rights and privileges," if you will, of being God, and became wholly dependent upon the spoken word and demonstrated will of the Father so that He might demonstrate how we should live our lives—in wholehearted devotion and dependence upon God (Matthew 4:4).

In the end, when we read of Jesus doing things that only God can do, and knowing things that only God can know, it's because at those moments, the Father was telling Him what to say, and showing Him what to do. When Jesus was asked about the time of His return, the Father did not give Him an answer…perhaps because He knew that they’d get complacent in their comfort or disheartened in their struggles if they knew that over 2,000 years into the future, we’d still be awaiting His glorious return as our King (Luke 19:27).