NQUIRING MINDS WANT TO KNOW: WHY AND WHEN DO WE SIT, STAND AND KNEEL AT MASS?   (Taken from the General Instruction of the Roman Missal #43)

The faithful should stand from the beginning of the Entrance Chant, or while the Priest approaches the altar, until the end of the Collect; for the Alleluia Chant before the Gospel; while the Gospel itself is being proclaimed; during the Profession of Faith and the Universal Prayer; and from the invitation (Pray, brethren), before the Prayer over the Offerings until the end of the Mass, except at the places indicated here below.

The faithful should sit, on the other hand, during the readings before the Gospel and the Responsorial Psalm and for the Homily and during the Preparation of the Gifts at the Offertory; and, if appropriate, they may sit or kneel during the period of sacred silence after Communion.

In the Dioceses of the United States of America, they should kneel beginning after the singing or recitation of the (Holy, Holy, Holy) until after the Amen of the Eucharistic Prayer, except when prevented on occasion by ill health, or for reasons of lack of space, or the large number of people present, or for another reasonable cause. However, those who do not kneel ought to make a profound bow when the Priest genuflects after the Consecration. The faithful kneel after the (Lamb of God) unless the Diocesan Bishop says otherwise.

If you would like information on why we stand or sit or kneel during the Mass, please check out the following websites:
www.formed.org: Explaining the Faith with Fr. Chris Alar – Part 6: Teaching Mass (Part 1); and Teaching Mass (Part 2)
www.eucharisticrevival.org: go to Blog: Do This in Remembrance of Me: A Step by Step Walk Through the Mass by Fr. Luke Spannagel (there are 52 Parts).