Ms. Grisham, the first lady's spokeswoman, dismissed curiosity about her travel, saying that it was "pretty typical" not to travel after a medical procedure.
But in the almost three weeks since then, Melania has forgone all public appearances and has not been seen by any members of the press as they cover the White House.
The surprise news comes after Mrs Trump failed to welcome reality TV star Kim Kardashian West to the Oval Office last week - a golden photo opportunity Mr Trump, 71, had to handle alone - and skipped a children's sports day on the White House lawns.
Ban plastic bags? UN seeks to cut pollution as recycling falls short
Leones said this should serve as a "wake up call" for the Philippines to reduce its plastic consumption. The UN is encouraging everyone to play a game of " plastic pollution tag ".
"I see the media is working overtime speculating where I am & what I'm doing", she wrote on Twitter.
Since then, the media and others have floated all kinds of theories about what might explain her brief inaccessibility.
It will be the first official event she has attended - and her first sighting - since May 10, when she was seen greeting the three American detainees newly freed by North Korea.
She is known for her privacy and White House independence, often breaking with her husband's schedule.
Grisham said the Gold Star families' event was important to Mrs. Trump and had been on her calendar for some time.
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Or someone has designed a robot that looks just like the first lady, at least from a distance. She hasn't made a public appearance since before her five-day hospital stay in mid-May for treatment of a kidney condition the White House only described as benign.
People are really wondering what Melania Trump is up to. In fact, many weren't convinced that Melania had been the one who'd sent the tweet. "She's assured in what she is doing and in her function, and is aware of the remaining is simply hypothesis and nonsense".
"We're used to our first ladies being out and about", Colbert added later.
But, wait. We can't let politics escape even a good story about the First Lady. Melania Trump won't be joining her husband at the 2018 G7 summit in Quebec or accompanying him to the meeting planned with North Korea's leader in Singapore following the G7.
Apple to unveil new iPhone, iPad and Mac features live
According to the firm, apps will launch up to 60% faster on an iPhone 6 Plus, a device that launched four years ago in 2014. In addition, there will be an Apple News app (currently available in iOS 11) and a refreshed App Store with video previews.