Trump also will be joined at the rally by his vice presidential running mate, Senator JD Vance, and the family of firefighter Corey Comperatore, who was fatally shot during the attempt on Trump's life.
Introductory speakers at the rally focused on remembering Comperatore.
"We will not let that tragic day overshadow our community's light," said JD Longo, mayor of the nearby town of Slippery Rock.
An attendee of the July rally, Shane Chesher, 37, said he would return on Saturday for what he expects will be an emotional event.
Chesher said he witnessed the assassination attempt from a seat on the stage behind Trump and is still processing what happened.
"Honestly, I don't know that I understand still to this day what we experienced that day," Chesher said in an interview.
Returning to the site, he said, "will be spiritual, and I think it will be very emotional, good and bad."
The Butler shooting led to widespread criticism of the U.S. Secret Service and the resignation of its director.
Critics raised concerns about how the 20-year-old suspect, Thomas Matthew Crooks, who was subsequently shot to death by Secret Service agents, was able to access a nearby rooftop with a direct line of sight to where Trump was speaking.
A Secret Service probe found communication gaps and a
lack of diligence ahead of the shooting. In its aftermath, the agency approved extra security measures for Trump, including using bulletproof glass to shield him at outdoor rallies.
Secret Service spokesperson Anthony Guglielmi said in a statement before Saturday's rally that there had been "comprehensive changes and enhancements to our communications capabilities, resourcing and protective operations."
Trump credits turning his head to read a chart on a big video as having saved his life. With blood dripping down his face, he raised a fist and shouted "fight" to his supporters, a vivid image from that day. He wore a white bandage on his wounded ear for days after the shooting.
Source: Reuters