Which days other than Sundays are catholics obliged to attend in mass?

Answer 1

Besides Sundays, Catholics are also obliged to attend Mass on Holy Days of Obligation. In the United States, the Holy Days of Obligation for Catholics of the Latin Rite are:

January 1, the solemnity of Mary, Mother of God;
Thursday of the Sixth Week of Easter, the solemnity of the Ascension;
August 15, the solemnity of the Assumption of the Blessed Virgin Mary;
November 1, the solemnity of All Saints;
December 8, the solemnity of the Immaculate Conception;
December 25, the solemnity of the Nativity of Our Lord Jesus Christ.

Whenever January 1, the solemnity of Mary, Mother of God, or August 15, the solemnity of the Assumption, or November 1, the solemnity of All Saints, falls on a Saturday or on a Monday, the precept to attend Mass is abrogated.

Answer 2

Yes, the first precept of the Church as contained in the Catechism of the Catholic Church is "To keep holy the day of the Lord's Resurrection: to worship God by participating in Mass every Sunday and Holy Day of Obligation: to avoid those activities that would hinder renewal of soul and body, e.g., needless work and business activities, unnecessary shopping, etc.

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To willing miss Mass on a Sunday or Holy Day of Obligation when one could have made it to Mass is a mortal (serious) sin and would necessitate going to confession before one could receive the sacraments again.