“To anyone beginning the study of Sunday and early Christian literature, the initial impression is one of confusion: Sunday is the first day, the day of creation, the day of light, the day of a new time. But it is also the last day, the eighth day, the day beyond days, the day of Jubilee, the day of the end time. It is the day of resurrection, but also the day of the postresurrection appearances and meals. It is the day of the dissent of the Spirit, day of the ascension, day of the assembly, day of the Eucharist, date of baptism, day of ordinations – until one asks, ‘Is there anything Sunday doesn’t mean?’ The answer, of course, is no. It was the symbolic day, sign of the time of the church between ascension and parousia, the time in which we are living now. It is the day symbolic of all days, for the purpose of our Christian liturgy is to express in a ritual moment that which should be the basic stance of every moment of our lives.” – Robert Taft, S.J.
So, basically, this is why you need to have your butts in church.