If Christians -- or Jews, or Muslims, or Wiccans, or Olympian pagans, or Satanists, or whoever -- want to hold their own "National Prayer Days", that's entirely their own business. Just don't impose it on the rest of us, or have government at any level having anything to do with it.
It has nothing to do with atheism. For most of their history, until the 1980s, evangelicals were among the strongest supporters of Jefferson and Madison's Wall of Separation. It's about government neutrality -- secularism -- in matters of individual conscience. Atheists have no more right to have government declare that there is no god than believers have for it to declare that there is.
<<Should a secular state have a National Prayer Day? Should a state without a requirement for its President to be Christian have a requirement that its President invoke Jesus?>>
US Constitution, Article VI:
"All Debts contracted and Engagements entered into, before the Adoption of this Constitution, shall be as valid against the United States under this Constitution, as under the Confederation.
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.
The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution;
but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States."
US Constitution, Amendment Article I:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.
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Or in short: NO!