Proclamation:

Proclamation is the title given to the action of Proclaiming a state, affirmation or declaration; it is a combination of two words, “pro” meaning forward, and “claim” meaning to state or assert that something is the case, sometimes done without providing evidence or proof.
Proclamation is usually used within an official formal public announcement. One of the functions of a proclamation is to bring in new “law” for members of the public.
Within government legislation, and within the legal system, for a new “law” to be created, first a policy must be drawn up and proposed, and then it must be proclaimed by someone to be “Law”.
Meaning within legislation a proclamation is required to bring in “colour of law”, a person must stand by and take the responsibility of this proclamation. It must be signed and dated.
Much of what is referred to as law within the legal system has not been proclaimed, and does not have a proclamation date as no one within government wants to be responsible for it.
Note: When someone attempts to apply legislation onto to you, it would be prudent to ask for the proclamation date and who did the proclaiming.
Within legislation these “colour of law” require proclamation to be enforceable, so they make you proclaim it by the action of opening the letter.