Yes, and no. Solemnities are special feasts – those greatly important to the Church and theology. All Sundays and holy days of obligation are solemnities, though not all solemnities are holy days of obligation.  Other holy days of obligation include the Nativity of Jesus (December 25), the Immaculate Conception of Mary (December 8), the Solemnity of the Ascension of Christ (Thursday of the sixth week of Easter), the Solemnity of the Assumption of Mary (August 15), and All Saints’ Day (November 1).