U.S. District Judge Barbara Crabb has ruled the May 6th observance unconstitutional.
The "Journal Sentinel" newspaper reports Crabb's decision calls the National Day of Prayer a violation of the First Amendment that prohibits laws establishing religion.
Crabb's ruling stems from action by the Freedom From Religion Foundation that had sued the Bush and Obama administrations from proclamations inviting Americans to set aside a day for prayer.
A White House spokesman said the ruling does not prevent President Obama from making his proclamation about the day this year.
Freedom From Religion Foundation co-President Anne Laurie Gaylor applauded the move by the Wisconsin judge.
Gaylor says the National Day of Prayer is "an invasion of the freedom of conscience of Americans." The National Day of Prayer was established in 1952 by President Harry Truman.
In 1988, Congress enacted legislation requiring the President to issue an annual proclamation declaring the first Thursday in May as National Day of Prayer.
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