With due respect, [user]Br. Rich SFO[/user] is not quite correct. What is fixed are:
Stand: From the entrance chant to the end of the Collect; from the *Alleluja *through the Gospel; during the Profession of Faith and the Prayers of the Faithful; from the *Orate Fratres *onward, except:
Sit: During the readings before the Gospel and the Responsorial Psalm; during the Homily; during the Offertory and the Preparation of the Gifts; the faithful may sit during the Sacred Silence following Communion.
Kneel: During the Consecration (except when prevented by health, large numbers of people, lack of space, or some other good reason).
The US (and, I believe, but am not sure: Canada) are among the many places that take advantage of an indult to kneel in variance of the above norms: "Ubi mos est, populum ab acclamatione Sanctus expleta usque ad finem Precis eucharisticae et ante Communionem quando sacerdos dicit Ecce Agnus Dei genuflexum manere, hic laudabiliter retinetur." (off the cuff translation) “Where it is the custom for the people to remain kneeling from the *Sanctus *to the end of the Eucharistic Prayer, and before Communion when the priest says Ecce Agnus Dei, this may be laudibly retained.”
tee