https://www.catholic.com/magazine/online-edition/what-are-the-holy-days-of-obligation
I've never heard of some of these "Holy Days". At the very least, they've renamed some it seems.
I'm rather surprised that you've not heard of all of these Holy Days, except for perhaps two which has been renamed. These were all important Holy Days before VII, though only six are "of obligation" in the United States. The bishops of each region have some say. For example, I was born in 1951 and have no recollection of St. Joseph (March 19) being of obligation in the United States, but it was a very important feast (Double of the First Class) and Mass attendance and Holy Communion were encouraged for those able. When I was in the Peace Corps in Colombia, St. Joseph (March 19) was a Holy Day of Obligation.Taking the list from the link and using for my reference "My Prayer Book" by Father F. X. Lasance, 1953 edition:the Nativity of our Lord Jesus Christ, December 25.
Also known as Christmas, this has always been a Holy Day of Obligation.
the Epiphany, January 6.
Fr. Lasance's doesn't list it' of obligation in the United States but of obligation in Canada pre VII.
the Ascension
Forty days after Easter. In most dioceses in the United it is observed on the Seventh Sunday of Easter rather than the preceding Thursday.
the Body and Blood of Christ
This may be one you don't recognize. The Feast of Corpus Christi (Thursday after Trinity Sunday) is combined with the Feast of The Most Precious Blood (July 1) and observed on the Thursday after Trinity Sunday in some countries, and on the Sunday after Trinity in the U.S. Both are Doubles of the First Class on the traditional Calander.
Holy Mary the Mother of God
This would be the second observance whose name you aren't familiar with. The Circuмcision on January 1 was renamed to this. The Gospel of the Mass is the same.
her Immaculate Conception
The same, December 8th, and still a day of obligation in the United States.
her Assumption
The same, August 15, and still a day of obligation in the United States.
Saint Joseph, March 19
Was not, and isn't now, a day of obligation in the United States, but is in some countries.
Saint Peter and Saint Paul the Apostles
Observed on June 29, as before. Not a day of obligation in the United States.
All Saints
A day of obligation in the United States, observed on November 1, as before.Those who follow a pre VII calendar and missal should recognize all of these names except for Holy Mary the Mother of God and the Body and Blood of Christ.