You can read it all here, compliments of Pastor Mark Surburg.
1) Good order, harmony and avoiding offense
Concerning church regulations made by human beings, it is taught to keep those that may be kept without sin and that serve to maintain peace and good order in the church, such as specific celebrations, festivals, etc. However, people are also instructed not to burden consciences with them as if such things were necessary for salvation. Moreover, it is taught that all rules and traditions made by human beings for the purpose of appeasing God and of earning grace are contrary to the gospel and the teaching concerning faith in Christ (AC XV.1-2).
We gladly keep the old traditions set up in the church because they are useful and promote tranquility, and we interpret them in an evangelical way, excluding the opinion which holds that they justify. Our enemies falsely accuse us of abolishing good ordinances and church discipline. We can truthfully claim that in our churches the public liturgy is more decent than theirs, and if you look at it correctly we are more faithful to the canons than our opponents are (Apology XV.38).
Nevertheless, we teach that liberty in these matters should be exercised moderately, so that the inexperienced may not take offense and, on account of an abuse of liberty, become more hostile to the true teaching of the gospel. Nothing in the customary rites may be changed without good reason. Instead, in order to foster harmony, those ancient customs should be observed that can be observed without sin or without proving to be a great burden. (Apology XV.51).
This is a simple rule for interpreting traditions. We should know that they are not required acts of worship, and yet we should observe them in their place and without superstition in order to avoid offense. This is the way many great and learned men in the church have felt about it (Apology XXVIII. 17-18).
2) Teaching the faith
But as the different length of day and night doe s not harm the unity of the church, so we believe that the true unity of the church is not harmed by differences in rites instituted by men, although we like it when universal rites are observed for the sake of tranquility. So in our churches we willingly observe the order of the Mass, the Lord’s day, and the other more important feast days. With a very thankful spirit we cherish the useful and ancient ordinances, especially when they contain a discipline that serves to educate and instruct the people and the inexperienced (Apology VII/VIII.33-34).
For after all, all ceremonies should serve the purpose of teaching the people what they need to know about Christ. (AC XXIV.3).
Ceremonies should be observed both so that people may learn the Scriptures and so that, admonished by the Word, they might experience faith and fear and finally even pray. For these are the purposes of the ceremonies. (Apology XXIV.3).And so when the confessors describe their own practice they say:
Among us the Mass is celebrated every Lord’s day and on other festivals, when the sacrament is made available to those who wish to partake of it, after they have been examined and absolved. We also keep traditional liturgical forms, such as the order of readings, prayers, vestments, and other similar things. (Apology XXIV.1)
Moreover, no noticeable changes have been made in the public celebration of the Mass, except that in certain places German hymns are sung alongside the Latin responses for the instruction and exercise of the people. (AC XXIV.3)