Objection 1. It would seem that promulgation is not essential to a law. For the natural law above all has the character of law. But the natural law needs no promulgation. Therefore it is not essential to a law that it be promulgated.
Objection 2. Further, it belongs properly to a law to bind one to do or not to do something. But the obligation of fulfilling a law touches not only those in whose presence it is promulgated, but also others. Therefore promulgation is not essential to a law.
Objection 3. Further, the binding force of a law extends even to the future, since "laws are binding in matters of the future," as the jurists say (Cod. 1, tit. De lege et constit. leg. vii). But promulgation concerns those who are present. Therefore it is not essential to a law.
On the contrary, It is laid down in the Decretals, dist. 4, that "laws are established when they are promulgated."
I answer that, As stated above (Article 1), a law is imposed on others by way of a rule and measure. Now a rule or measure is imposed by being applied to those who are to be ruled and measured by it. Wherefore, in order that a law obtain the binding force which is proper to a law, it must needs be applied to the men who have to be ruled by it. Such application is made by its being notified to them by promulgation. Wherefore promulgation is necessary for the law to obtain its force.
Thus from the four preceding articles, the definition of law may be gathered; and it is nothing else than an ordinance of reason for the common good, made by him who has care of the community, and promulgated.
Reply to Objection 1. The natural law is promulgated by the very fact that God instilled it into man's mind so as to be known by him naturally.
Reply to Objection 2. Those who are not present when a law is promulgated, are bound to observe the law, in so far as it is notified or can be notified to them by others, after it has been promulgated.
Reply to Objection 3. The promulgation that takes place now, extends to future time by reason of the durability of written characters, by which means it is continually promulgated. Hence Isidore says (Etym. v, 3; ii, 10) that "lex [law] is derived from legere [to read] because it is written."
Source: Summa Theologica (New Advent)