What is a day of obligation?
That is a day that you are supposed to go to Church. In the 1928 BCP they are defined as follows.
A TABLE OF FEASTS,
TO BE OBSERVED IN THIS CHURCH THROUGHOUT THE YEAR.
All Sundays in the Year
St. Bartholomew the Apostle
The Circumcision of our Lord Jesus CHRIST
St. Matthew, Apostle and Evangelist
The Epiphany
St. Michael and all Angels
The Conversion of St. Paul
St. Luke the Evangelist
The Purification of the Blessed Virgin
St. Simon and St. Jude, Apostles
St. Matthias the Apostle
All Saints
The Annunciation of the Blessed Virgin
St. Andrew the Apostle
St. Mark the Evangelist
St. Thomas the Apostle
St. Philip and St. James. Apostles
The Nativity of our Lord JESUS CHRIST
The Ascension of our Lord JESUS CHRIST
St. Stephen, Deacon and Martyr
St. Barnabas the Apostle
St. John. Apostle and Evangelist
The Nativity of St. John Baptist
The Holy Innocents
St. Peter the Apostle
Monday and Tuesday in Easter Week
St. James the Apostle
Monday and Tuesday in Whitsun Week
The Transfiguration of our Lord JESUS CHRIST