When Does Transubstantiation Occur?

[brianthephysicist]

Growing up (and still to this day) I heard any number of lay, religious, and priests disagree on this issue.  I have been told that it occurs: at the Words of Institution (This is my Body...This is the chalice of my Blood), at the epiclesis, in stages during the Eucharistic prayer, at an unspecified time during the Eucharistic prayer, in stages during the entire Mass, and at an unspecified time during the Mass.  Each of these has been accompanied by its own "proof", which some seem quite convincing and others seem quite flimsy.  

 

Can you help me find where transubstantiation occurs?  

 

Even if you can't make a bold declarative statement to choose precisely 1 of these options (or one I might not have even heard yet), can you help by ruling out the ones that are obviously false?

 

Thanks guys, you're the best.

[cappie]

It is an active discussion however the Catechism of the Catholic Church says:

 

Transubstantiation occurs during the consecratory thanksgiving during the single act of worship called the liturgy of the Eucharist.

1377 The Eucharistic presence of Christ begins at the moment of the consecration and endures as long as the Eucharistic species subsist.
http://www.vatican.va/archive/ENG0015/__P41.HTM

During Mass the liturgy of the Eucharist begins a particular time and follows this structure:

1346 The liturgy of the Eucharist unfolds according to a fundamental structure which has been preserved throughout the centuries down to our own day. It displays two great parts that form a fundamental unity: - the gathering, the liturgy of the Word, with readings, homily and general intercessions; - the liturgy of the Eucharist, with the presentation of the bread and wine, the consecratory thanksgiving, and communion.
The liturgy of the Word and liturgy of the Eucharist together form "one single act of worship";170 The Eucharistic table set for us is the table both of the Word of God and of the Body of the Lord.
http://www.vatican.va/archive/ENG0015/__P40.HTM

The consecratory thanksgiving is the moment when the priest holds up the bread and repeats the words of Christ: "This is my body ..." and so on. Likewise, he says the same for the wine "This is the cup ... " and so on.

1412 The essential signs of the Eucharistic sacrament are wheat bread and grape wine, on which the blessing of the Holy Spirit is invoked and the priest pronounces the words of consecration spoken by Jesus during the Last Supper: "This is my body which will be given up for you.... This is the cup of my blood...."
1413 By the consecration the transubstantiation of the bread and wine into the Body and Blood of Christ is brought about.
http://www.vatican.va/archive/ENG0015/__P44.HTM