The answer
The transubstantiation of the holy mysteries takes place when the Holy Spirit descends, and not before.
And the descent of the Holy Spirit takes place just before the Intercessions and the commemoration of the saints. So the priest prays in secret saying: "Let Your Holy Spirit descend upon us and upon these sacrifices placed here. Purify them and transform them and make them to appear holy to your saints ".
Then he makes the sign of the cross over the bread three times and calls aloud: 'He makes this bread His holy body'. Then he makes the sign of the cross three times over the chalice and, calls aloud: '.. and this cup too, He makes the honoured blood of the New Testament...' and the people say 'Amen' after both of these.
This proves that no transformation takes place during the offering of the Host.
For if the mysteries were transformed before this point, the priest would not call for the Holy Spirit to descend to change them.
We also observe that after the descent of the Holy Spirit to transform the sacraments, the priest does not make the sign of the cross, and does not look behind him.
Before that - after the offering of the Host and the signing of the cross - the priest makes the sign of the cross over the people, and the bread and wine. However, after its transubstantiation, when the Holy Spirit has descended, he does not make the sign of the cross over the people any more, not even when he says, 'Peace be with you all'. In fact he just bows his head without signing the cross.
Nor does he sign the cross over the chalice or the offertory paten (tray), but makes the sign of the cross with each of the sacraments in turn, after their transubstantiation, over the other one.
This means that the blood is crossed by the body, and the body by the blood, but the priest doesn't do it with his hand or finger at all.
He does not turn round to face the people at all when he blesses them but instead focuses his concentration on the holy mysteries, without turning away from them.
From this one can see that to say that the mysteries are transformed directly after the offering of the Host during the first signing of the cross, is inaccurate. If it were so, then it would mean that the mysteries are sanctified and transformed, during the part of the Mass that is attended by the catechumens for those are not allowed to attend the whole Mass.
But we observe that in the early days of the Church, the catechumens used to attend the offering of the Host and the reading of the epistles and the gospel and the sermon, and would then depart. And the deacon, before the raising of the holy veil concealing the sacraments, meaning before the Holy Mass was begun, would say: 'Let no-one who is unconfirmed or who is an unbeliever stand here, but let only those believers stay who are worthy to attend the divine Mass'. (See the canons of the apostles and those of Apolides.)
Studying the history of the Church's ceremonies calls for a knowledge of theology of the rituals involved and also their spirituality.
Since history does not conflict with theology, we can see that it cannot be said that in the past, the holy mysteries used to be transformed from the bread and wine into the body and blood of the Lord, before the descent of the Holy Spirit upon them and the prayers of the priest invoking this descent.
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Thrice-Blessed Pope Shenouda III of Blessed Memory