As far as I know, the practice of the Lord's table is practiced once a month by most Protestant churches to be something either practiced for such a long time that it is an unwritten rule. It is just common practice, perhaps seen in the same manner as two opposite sex couples who after living together for years with children and never been married are by some states considered common-law marriage or cohabitation as legal proof of marriage even without a ceremony or certificate. There might be a written rule in the by-laws or even within the perameters of how to conduct the service itself.