1st Week After Octave of Easter. From "The Roman Breviary" - at Matins

Here begins the book of the Acts of the Apostles

Lesson i: c.I, 1-26
The former treatise I made, O Theophilus, of all things which Jesus began to do and to teach, until the day on which, giving commandments by the Holy Ghost to the apostles whom he had chosen, he was taken up. To whom also he shewed himself alive after his passion, by many proofs, for forty days appearing to them and speaking of the Kingdom of God. And eating together with them, he commanded them that they should not depart from Jerusalem, but should wait for the promise of the Father, which you have heard (saith he) by my mouth. For John indeed baptized with water, but you shall be baptized with the Holy Ghost, not many days hence. They therefore who were come together asked him, saying: Lord, wilt thou at this time restore again the kingdom to Israel? But he said to them: It is not for you to know the times or moments, which the Father hath put in his own power: but you shall receive the power of the Holy Ghost coming upon you, and you shall be witnesses unto me in Jerusalem, and in all Judea and Samaria, and even to the uttermost part of the earth.

Lesson ii
And when he had said these things, while they looked on, he was raised up: and a cloud received him up to heaven, behold two men stood by them in white garments. Who also said: Ye men of Galilee, why stand you looking up to heaven? This Jesus who is taken up from you into heaven shall so come as you have seen him going into heaven. Then they returned to Jerusalem from the mount that is called Olivet, which is nigh Jerusalem, within a sabbath day's journey. And when they were come in, they went up into an upper room, where abode Peter and John, James and Andrew, Philip and Thomas, Bartholomew and Matthew, James of Alfeus and Simon Zelotes and Jude the brother of James. All these were persevering with one mind in prayer with the women and Mary the mother of Jesus, and with his brethren.

From "The Saint Andrew Daily Missal" - Historical note for Easter
....I went forth from the Father and am come into the world: again I leave the world and go to the Father. Rejoice, for I go to prepare a place for you in heaven, so that there where I am, you may be also. Yet a little while and you shall see me no more, and then you shall be plunged into sadness. But I will not leave you fatherless, I will come to you again through my Holy Spirit, for if anyone love Me in this Holy Spirit, my Father will love him and We will come to him and make our abode with him. I will therefore, ask My Father to send you the Holy Ghost, and you will then rejoice always. And when this Holy Spirit is come, He will give testimony of Me and you will then ask the Father in My name (that is, resting on my merits all the efficacy of which you will then understand, for I have strengthened you that you may go forth and bear fruit). I am the Vine, you are the branches. He who abideth in Me and I in him, he bringeth forth much fruit. And you shall be purged that you may bring forth still more, for as the world has persecuted me, so it will persecute you also. But fear not, for the Holy Ghost will speak by you and by the mouth He will convict the world of sin, showing by you that with Satan it is already judged, because it has rejected Him whom the the Father sent and glorified (Ressurection and Ascension), and that in rejecting the Son it has rejected the Father, for he sees also the Father who sees the Son.