The reasons for Holy Days of Obligation dates back to the Sacred Tradition practiced from the recorded history from the book of Acts 2:42-46, 1Corinthians 11:17 and Hebrews 10:25 "We should not stay away from our assembly, as is the custom of some, but encourage one another, and this all the more as you see the day drawing near"...
Outside of the Holy Day of obligation on the first day "Sunday", the Church can abolish certain days of obligation or transfer them to a Sunday from her binding and loosing authority given to her as Jesus binds and looses in heaven.
The obligation of holy days falls under the precepts of the Church, which are disciplines that can change. They are set in the liturgical calender to celebrate the events of Jesus Christ in our humanity (birth, Epiphany, Ascension and his body and blood of Christi) which include those who eye witnessed Jesus, Mary the blessed Virgin Mary, ST. Joseph and St's. Peter and Paul, along with those who gave their life as a witness to Jesus in the feast of "All Saints day".
These Holy Days are set as a minimum (discipline) in the spirit of prayer and moral effort to grow in the Love of God and neighbor throughout the year. Some of these Holy Days include disciplines of fasting.
In these Holy Day celebrations includes a Mass which celebrates the death and resurrection of our Lord Jesus Christ Truly present in his body, blood soul and divinity in our midst.
It should be stated that the Mass is celebrated every day from the rising of the sun to its setting (Malchi 1:11) world wide today. The Holy Days of obligation invites "all" believers to the banquet of the Lord.
These Holy Days help us "to pray without ceasing"