are there holy days of obligation in GO faith? If so same as Catholics? Is Sunday in GO also an obligation? Thanks
By GO are you referring to the Greek Orthodox? If so I’ll try to give an answer since I’m Orthodox (Antiochian).
There are not days of obligation as such on the penalty of mortal sin in Orthodoxy. One is however expected to attend liturgy each Sunday and on important feast days, and this is taken seriously. I was advised by my godfather as a catechumen that in the early church if one missed more than three consecutive Sunday liturgies without good cause then he was considered to have excommunicated himself! My wife is Catholic, and our attendance of services is an issue of some concern for me because we usually attend at my parish now since I’m an altar server and need to be there each week. Previously we would attend Orthodox liturgies on week, and Catholic ones the next. If I remember correctly your daughter is dating an Orthodox man, so this is an area they’ll need to address. One option would be Catholic Mass on Saturday evenings (Orthodox generally have Vespers then, but not Divine Liturgy).