There was actually a series of eight steps between Advent Sunday, 1964 and Advent Sunday, 1969. I wrote a series of articles about this 17 years ago, largely pulled from the archives of the archdiocese where I lived at the time. When the first changes started on Advent Sunday, 1964, the first transitional Missal that many here are thinking about–no one really expected there to be as many changes as there would eventually be.
Three separate altar missals were published in the US during the transition period, and I have each of them in my library:
- The first in 1964, reflected Inter Oecumenici, which was the Instruction on the implementation of the Constitution on the Sacred Liturgy.
- The second in 1966, reflected instructions from the Holy See regarding additional reforms in 1965, and the US bishops’ vote to increase the use of the vernacular to all audible prayers in the Mass.
- The third in 1968, reflected Tres Abhinc Annos, which was the Second Instruction on the implementation of the Constitution on the Sacred Liturgy in 1967, and the addition of three new Eucharistic Prayers in 1968.
At the time, since the reform of the Roman Missal was underway, and as yet still not complete, the Order of Mass was being referred to by the Holy See as the Novus Ordo Missae, i.e., the new Order of Mass. This term was still widely used, including by Blessed Paul VI himself in his weekly addresses, for the first several years of the reformed Missal, since various countries were implementing their translations at varying speeds. This was the origin of the term Novus Ordo, and not anything negative.**
However, the use of that term has become quite negative over the years, as explained here
forums.catholic-questions.org/showpost.php?p=11478809&postcount=47