This practice of celebrating Mass - technically not with my back to you, we're all facing the same direction - celebrating Mass ad orietum, or "toward the East" comes from this day, from this Feast.
When our Lord ascended into Heaven, all of the apostles and disciples were facing East. And that's where our Lord ascended, toward the East.
We know that they stood there after He was gone and watched for a little while, and what happened? Two men dressed in white, we know they're angels, appeared to them and said, "Men of Galilee, what are you looking at?"
Now, they're angels. Obviously they knew what the men were looking at. They weren't seeing Jesus anymore, He was gone. The angels tell them basically: don't worry, He's going to come again in the same way that He is gone from your sight.
These angels, these messengers form God, were the ones instructed to inform the apostles that when Jesus comes in glory, He will come from the East on the clouds of Heaven. From the East.
Ad orientum. Toward the East.
So even though they didn't know when Jesus was going to come, every time they gathered together in prayer to offer the Holy sacrifice of the Mass, all of them - including the presider - would face East, just in case Jesus would come.
For two thousand years in the Church that has been the tradition. Every liturgy. Always. The presider - whether it's the Pope, Bishops or Priests - and the people would face the same direction just in case the Lord would come, so that they were ready to see Him.
That's where this tradition came from, and that's why we've maintained it for two thousand years.
Once I was ordained, I began to appreciate, as the celebrant, how important this is.
"Seeing"in any type of temporal vision is not necessary because our eyes are not fixed on time and space, but on Heaven. On eternity. We are waiting for the return of the Lord.
This is the beauty of ad orietum. It daily reminds us to look towards the heavens and to contemplate the return of our Lord.