JMJ

Saints days and feast days fall into a number of categories.  The highest category is called a Solemnity.  There are 15 Solemnities on the Church calendar; they are:
Jan.   1: The Solemnity of Mary, Mother of God (Theotokos)
Jan.   6: Epiphany
Mar. 19:  St. Joseph, Husband of Mary
Mar. 25: Annunciation
--------     Holy Trinity [First Sunday after Pentecost]
--------     Corpus Christi [Thursday after Holy Trinity]
--------     Sacred Heart of Jesus [Friday after Second Sunday
               after Pentecost
--------     Immaculate Heart of Mary [Saturday after the Second
               Sunday after Pentecost
Jun. 24   Birth of John the Baptist
Jun. 29   Sts. Peter and Paul
Aug. 15  Assumption of Mary
Nov.  1   All Saints
---------   Christ the King [Last Sunday of the Liturgical Year]
Dec.  8   Immaculate Conception of Mary
Dec. 25  Christmas


At the next level are the 25 Feast Days:
Jan. 25   Conversion of St. Paul
--------    Baptism of the Lord [Sunday after Jan. 6]
--------    Holy Family [Sunday within the Octave of Christmas]
Feb.  2   Presentation of the Lord
Feb. 22   Chair of Peter
Apr. 25   St. Mark, the Evangelist
May  3   Philip and James, Apostles
May 14  Matthias, Apostle
May 31  Visitation of Mary to Elizabeth
Jul.    3   St. Thomas, Apostle
Jul.  25   St. James, Apostle
Aug.  6   Transfiguration
Aug. 10  St. Lawrence
Aug. 24  Bartholomew, Apostle
Sep.   8  Birth of Mary
Sep. 14  Triumph of the Holy Cross
Sep. 21  Matthew, Apostle and Evangelist
Sep. 29  Michael, Gabriel, Raphael, Archangels
Oct. 18  Luke, Evangelist
Oct. 28  Simon and Jude, Apostles
Nov.  9  Dedication of St. John Lateran
Nov. 30  Andrew, Apostle
Dec. 26  Stephen, the first Martyr
Dec. 27  John, Apostle and Evangelist
Dec. 28  Holy Innocents


At the third level are Memorials and at the fourth are Optional Memorials.
Not all Solemnities are Holy Days of Obligation but all Holy Days of Obligation are Solemnities.  Every Sunday is considered a Solemnity and could be regarded as a Holy Day of Obligation.
The calendar was revised after Vatican II and the memorials and feasts in the new calendar are called memorials and feasts of the Novus Ordo.  Sundays and feasts  before 1962 are called "Old" or "Traditional."  When a Latin Mass is celebrated, the "Old" or "Traditional" calendar is used.  The lowest category consists of "Historical" saints.  These are saints who have been removed from the Universal Calendar altogether, such as St. Christopher, St. Catherine of Alexandria, St. Philomena, St. Dismas (the Good Thief), St. Gemma Galgani, and St. Dominic Savio, and so forth.  These saints used to have memorials.  On the other hand, they remain as saints of the Church and their lives and examples can and should also be studied and emulated.

May the Peace of Christ always be with you.