This is the day which the Lord has made. Let us rejoice and be glad in it. Give thanks to the Lord, for he is good; his mercy endures forever.

The reading is from the Acts of the Apostles. Chapter 1:1-8

In the first book, O Theophilus, I have dealt with all that Jesus began to do and teach, until the day when he was taken up, after he had given commandment through the Holy Spirit to the apostles whom he had chosen. To them he presented himself alive after his passion by many proofs, appearing to them during forty days, and speaking of the kingdom of God. And while staying with them he charged them not to depart from Jerusalem, but to wait for the promise of the Father, which, he said, "you heard from me, for John baptized with water, but before many days you shall be baptized with the Holy Spirit."

So when they had come together, they asked him, "Lord, will you at this time restore the kingdom of Israel?" He said to them, "It is not for you to know times or seasons which the Father has fixed by his own authority. But you shall receive power when the Holy Spirit has come upon you; and you shall be my witnesses in Jerusalem and in all Judea and Samaria and to the end of the earth."