Marty, I gather you are referring to, "yet we are not bound to or tied down to any particular day." But as I understand it in its native context, he is speaking about "that which is particular" so far as Sabbath sanctification is concerned. Hence his meaning is only that there is no inherent moral holiness placed in one day over another, so that the day is to be considered one of positive appointment. The Puritans held the same. As for the appointment of the first day of the week, he provides a significant reason why this day was appointed over others, namely, "because on that day the resurrection of Christ took place, by which the internal and spiritual Sabbath is begun in us." (P. 563.) Again, the Puritans held the same. It is also worth noting that Ursinus would use the word "church" in the context of "Christendom," or the idea of a universal church, not in any sense suggesting that it is up to each individual congregation to decide what day it will worship on. Blessings!