I come from a Roman Catholic background where we had holy days of obligation, which meant if you were not in church on those days, you committed a mortal sin. I know Orthodoxy does not believe in mortal sin (a relief!), but besides major feast days (Pascha, the Incarnation, etc), are there any other holy days where you're obligated to attend church?
My understanding is that, in addition to Pascha, you are also supposed to attend on the 12 great feasts, at least either on the vigil or for the feast, provided that you are able and the parish has services. Some of these always fall on Sunday and the rest, except Ascension, may sometimes fall on Sunday.
Nativity of the Theotokos (Sept 8/21)
Exaltation of the Cross (Sept 14/27)
Presentation of the Theotokos (Nov 21/Dec 4)
Nativity of Our Lord (Dec 25/Jan 7)
Theophany (Jan 6/19)
Presentation of Our Lord (Feb 2/15)
Annunciation (Mar 25/Apr
Ascension of Our Lord
Transfiguration (Aug 6/19)
For motion of the Mother of God (Aug 15/28)
To these you should add the other great feasts
Circumcision of Our Lord (Jan 1/14)
Nativity of Saint John the Baptist (June 24/July 7)
Saints Peter and Paul (June 29/July 12)
Beheading of Saint John the Baptist (August 29/Sept 11) kept as a very strict fast
Protection of the Mother of God (Oct 1/14)
The name day of your parish is said to be second in rank only to Pascha and everyone should make a good effort to be there.