And Tucker was wrong. Nothing in Washington's proclamation was mandatory, it was simply a suggestion. Tucker himself forgot the words he wrote just a few sentences before, "Liberty of conscience in matters of religion consists in the absolute and unrestrained exercise of our religious opinions, and duties, in that mode which our own reason and conviction dictate, without the control or intervention [involvement] of any human power or authority whatsoever."
The Constitution does not prohibit the President from making such proclamations as he pleases. The president has the Liberty of Conscience guaranteed by the Constitution and as long as he does not compel any person to pray or fast, he can call for prayer and fasting anytime he so pleases. Those who are like minded will pray and fast and those who are not like minded are free to ignore the proclamation or to vote the guy out of office.
The power of the executive branch is delegated to a single man. That man does not give up his freedom of speech or his freedom of religion by simply taking an oath to uphold and defend the constitution of the United States. He is certainly free to ask America to join him in fasting and prayer. To take that away from the President is to take the presidency out of the hands of the man. That would be unconstitutional.