WHAT DAY WAS IT THAT MASS BEGAN TO BE CELEBRATED FACING THE PEOPLE?

Actually, in many places in the world, the pre-Vatican II Mass was celebrated facing the people. It wasn’t a Vatican II innovation. 
Was it wise? Perhaps with the Tridentine Mass it was not a bad idea if one maintained the proper altar arrangement and the rubrics of the Mass although some rubrics had to be adjusted if the priest faced the nave. 
This is the section from the Adoremus article on Mass facing the people. It is quite informative:

Many Catholics think that Mass facing the people was an innovation of Vatican II. However, liturgists had been arguing for decades that it was permitted by the rubrics of the older Missal.

For example, in 1937 Orate Fratres published a question about an earlier article that mentioned a Mass said by a priest facing the congregation, and asked about the justification for this practice. The response said that it was the current custom in St. Peter’s Basilica and other Roman churches for the priest to face the people because of the location of the altar, and that there were specific rubrics for the Mass instructing the priest what to do in such cases. It concluded:

Not only, therefore, does no Church law or rubric forbid the construction of altars at which the celebrant faces the people as of old, but the present rubrics, as quoted above, still make provision for Mass celebrated at such an altar. (Orate Fratres, April 18, 1937, p. 280.)

In 1959, the same journal, by now known as Worship, addressed a variation of the same question (vol. 33, #2, pp. 123-125). In this article the question was whether a temporary altar could be placed in front of the old altar so that the priest could say Mass facing the people.

This question was answered by Father Frederick McManus, who taught canon law at Catholic University and was wellknown as a speaker and writer on liturgical law. In his response, Father McManus said that a temporary altar was perfectly lawful, and suggested that such an altar should be close to the Communion rail. He also said that a permanent altar facing the people may be erected, and cites an article of Father Annibale Bugnini explaining that the desire to have Mass facing the people is a justification for reserving the Blessed Sacrament on an altar other than the main altar. He says that the Holy See is “encouraging” a “revival” of Mass facing the people; but the evidence he cites is less conclusive. He notes that at an international conference on liturgy held in Assisi in 1956, Pope Pius XII had said that experts would study the question of what should be done with the tabernacle on an altar facing the people; and he also cites a 1957 decision from the Congregation for Rites that said that if there is only one altar in a church it cannot be used for Mass facing the people. Father McManus sees this decision as permissive, because it does not forbid the practice as such, but deals only with the case of a church with a single altar — a situation that he considers rare. Many Catholics think that Mass facing the people was an innovation of Vatican II. However, liturgists had been arguing for decades that it was permitted by the rubrics of the older Missal.

For example, in 1937 Orate Fratres published a question about an earlier article that mentioned a Mass said by a priest facing the congregation, and asked about the justification for this practice. The response said that it was the current custom in St. Peter’s Basilica and other Roman churches for the priest to face the people because of the location of the altar, and that there were specific rubrics for the Mass instructing the priest what to do in such cases. It concluded:

Not only, therefore, does no Church law or rubric forbid the construction of altars at which the celebrant faces the people as of old, but the present rubrics, as quoted above, still make provision for Mass celebrated at such an altar. (Orate Fratres, April 18, 1937, p. 280.)

In 1959, the same journal, by now known as Worship, addressed a variation of the same question (vol. 33, #2, pp. 123-125). In this article the question was whether a temporary altar could be placed in front of the old altar so that the priest could say Mass facing the people.

This question was answered by Father Frederick McManus, who taught canon law at Catholic University and was wellknown as a speaker and writer on liturgical law. In his response, Father McManus said that a temporary altar was perfectly lawful, and suggested that such an altar should be close to the Communion rail. He also said that a permanent altar facing the people may be erected, and cites an article of Father Annibale Bugnini explaining that the desire to have Mass facing the people is a justification for reserving the Blessed Sacrament on an altar other than the main altar. He says that the Holy See is “encouraging” a “revival” of Mass facing the people; but the evidence he cites is less conclusive. He notes that at an international conference on liturgy held in Assisi in 1956, Pope Pius XII had said that experts would study the question of what should be done with the tabernacle on an altar facing the people; and he also cites a 1957 decision from the Congregation for Rites that said that if there is only one altar in a church it cannot be used for Mass facing the people. Father McManus sees this decision as permissive, because it does not forbid the practice as such, but deals only with the case of a church with a single altar — a situation that he considers rare. Many Catholics think that Mass facing the people was an innovation of Vatican II. However, liturgists had been arguing for decades that it was permitted by the rubrics of the older Missal.

For example, in 1937 Orate Fratres published a question about an earlier article that mentioned a Mass said by a priest facing the congregation, and asked about the justification for this practice. The response said that it was the current custom in St. Peter’s Basilica and other Roman churches for the priest to face the people because of the location of the altar, and that there were specific rubrics for the Mass instructing the priest what to do in such cases. It concluded:

Not only, therefore, does no Church law or rubric forbid the construction of altars at which the celebrant faces the people as of old, but the present rubrics, as quoted above, still make provision for Mass celebrated at such an altar. (Orate Fratres, April 18, 1937, p. 280.)

In 1959, the same journal, by now known as Worship, addressed a variation of the same question (vol. 33, #2, pp. 123-125). In this article the question was whether a temporary altar could be placed in front of the old altar so that the priest could say Mass facing the people.

This question was answered by Father Frederick McManus, who taught canon law at Catholic University and was wellknown as a speaker and writer on liturgical law. In his response, Father McManus said that a temporary altar was perfectly lawful, and suggested that such an altar should be close to the Communion rail. He also said that a permanent altar facing the people may be erected, and cites an article of Father Annibale Bugnini explaining that the desire to have Mass facing the people is a justification for reserving the Blessed Sacrament on an altar other than the main altar. He says that the Holy See is “encouraging” a “revival” of Mass facing the people; but the evidence he cites is less conclusive. He notes that at an international conference on liturgy held in Assisi in 1956, Pope Pius XII had said that experts would study the question of what should be done with the tabernacle on an altar facing the people; and he also cites a 1957 decision from the Congregation for Rites that said that if there is only one altar in a church it cannot be used for Mass facing the people. Father McManus sees this decision as permissive, because it does not forbid the practice as such, but deals only with the case of a church with a single altar — a situation that he considers rare.