The rally will coincide with the annual White House correspondents' dinner, which Trump elected not to attend in a break from tradition.
The 100th-day mark has traditionally been an early measuring point for a new president's effectiveness.
Trump has been eager to show progress on his agenda but so far has had no major legislative victories. In another tweet Saturday, Trump echoed what he told the Associated Press in an interview a day earlier that he plans to make an announcement related to a new tax proposal Wednesday. But he and his aides have offered no details.
Trump's rally would offer counterprogramming to the annual celebration of political journalism that draws more than 2,000 reporters, celebrities and advertisers to the Washington Hilton. Trump announced in February that he would be the first president in 36 years to skip the dinner. His staff also turned down invitations to be guests at the event.
Trump has had a contentious relationship with the press corps, whom he has labeled "fake news," and some news organizations have canceled their parties that weekend. But the White House Correspondents' Association has said the dinner will continue, citing the mission of raising money for student scholarships and promoting the journalism of the White House press corps and other independent journalists.
Comedian Hasan Minhaj of Comedy Central's "The Daily Show" will be the featured performer at the dinner. Reporters Bob Woodward and Carl Bernstein are scheduled to present the journalism awards.