In a Twitter message Saturday morning, the president said he would be "holding a BIG rally in Pennsylvania" on April 29, the same night the press corps will be gathering for its annual black-tie event at the Washington Hilton.
The announcement came a day after Trump dismissed the importance of the 100-day mark as a "ridiculous standard". Trump previously announced that he is boycotting this year's dinner to protest what he says is unfavorable coverage by the news media.
The White House did not offer additional details about where the rally would take place or what Trump's focus would be, referring questions to Trump's campaign apparatus.
A senior White House official said the decision to hold the rally that day had more to do with how Trump planned to spend the hours of the 100th day than the correspondents' dinner.
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The 100 days measure is linked to President Franklin Roosevelt, who signed 15 major pieces of legislation early in his presidency; the time frame is when a president's power and influence with Congress is likely the greatest.
Mr. Trump didn't elaborate on what the rally in Pennsylvania will entail.
April 29 is also the date of the annual White House Correspondents' Association dinner in Washington.
Republican presidential nominee Donald Trump walks to the stage at a campaign rally in Pensacola, Florida, U.S., September 9, 2016.