81 The gift of the Holy Spirit, sent upon His disciples, is the first notable sign of His abounding charity after His triumphant ascent to the right hand of His Father. For after ten days the Holy Spirit, given by the heavenly Father, came down upon them gathered in the Upper Room in accordance with the promise made at the Last Supper: "I will ask the Father and He will give you another Paraclete so that He may abide with you forever."(84) And this Paraclete, who is the mutual personal love between the Father and the Son, is sent by both and, under the adopted appearance of tongues of fire, poured into their souls an abundance of divine charity and the other heavenly gifts.
Source: Haurietis Aquas (Vatican.va)