Archbishop Lefebvre was very clear and succinct on attendance at the New Mass - here are only a few of his statements on this subject:
“It must be understood immediately that we do not hold to the absurd idea that if the New Mass is valid, we are free to assist at it. The Church has always forbidden the faithful to assist at the Masses of heretics and schismatics even when they are valid. It is clear that no one can assist at sacrilegious Masses or at Masses which endanger our faith.…All these innovations are authorized. One can fairly say without exaggeration that most of these [new] Masses are sacrilegious acts which pervert the Faith by diminishing it. The de-sacralization is such that these Masses risk the loss of their supernatural character, their mysterium fidei; they would then be no more than acts of natural religion. These New Masses are not only incapable of fulfilling our Sunday obligation, but are such that we must apply to them the canonical rules which the Church customarily applies to communicatio in sacris with Orthodox Churches and Protestant sects.” (The New Mass and the Pope, November, 8, 1979)
“Your perplexity takes perhaps the following form: may I assist at a sacrilegious Mass which is nevertheless valid, in the absence of any other, in order to satisfy my Sunday obligation? The answer is simple: these Masses cannot be the object of an obligation; we must moreover apply to them the rules of moral theology and Canon Law as regards the participation or the attendance at an action which endangers the faith or may be sacrilegious. The New Mass, even when said with piety and respect for the liturgical rules, is subject to the same reservations since it is impregnated with the spirit of Protestantism. It bears within it a poison harmful to the faith.” (An Open Letter to Confused Catholics, Ch. 4)
“This Mass is poisoned, it is bad and it leads to the loss of faith little by little. We are clearly obliged to reject it.” (The Mass of All Times, p. 353)