According to the website, a presidential proclamation is
...“an instrument that states a condition, declares a law and requires obedience, recognizes an event or triggers the implementation of a law (by recognizing that the circumstances in law have been realized)” (Cooper 2002, 116). In short, presidents “define” situations or conditions on situations that become legal or economic truth. These orders carry the same force of law as executive orders – the difference between the two is that executive orders are aimed at those inside government while proclamations are aimed at those outside government. The administrative weight of these proclamations is upheld because they are often specifically authorized by congressional statute, making them “delegated unilateral powers.” Presidential proclamations are often dismissed as a practical presidential tool for policy making because of the perception of proclamations as largely ceremonial or symbolic in nature. However, the legal weight of presidential proclamations suggests their importance to presidential governance.
Now, the database goes back to 1789 with G.W., who issued one proclomation that seems to be a formal recognition of Thanksgiving as being November 26th. In 1790 he makes two proclomations pertaining to treaties reached with Indian tribes. In 2009, there are several proclamations per month, most of which seem to be raising awareness of something. Just skipping around, it seems this behavior started with Ford in 1974-1975. Prior to Ford, nobody seems to issue a proclamation on even a monthly basis.
It would be interesting to try an correlate number of proclamations with employment or other business cycle variables.