A day prior to the performance, a source dished to ET about why the couple wanted John -- a friend of Harry and Prince William's mother, Princess Diana -- to take part in their special day.
"It is no surprise that Elton has been included in Prince Harry's wedding. As one of Diana's closest confidantes, Elton is a connection to Harry's mother and a very supportive friend," the source said of the music legend. "The plan is for Elton to play at the wedding reception has been in place for a while."
Added the source, "The wedding itself will be very formal and the reception is meant to be more fun, and that is where Elton comes in."
The source also noted that John was expected to mention Diana during his performance.
"It is thought that the 71-year-old musician will also honor Harry's mother on Saturday," the source said. "He plans to sing and there's also been talk of some type of tribute to Diana, one in which Elton will speak at the wedding and say how much she is missed on this very special day. [Elton] has remained very close to her sons."
In a recent piece for Time magazine, John let it be known that he has much respect for Harry. "Prince Harry has that rare ability to walk into a room full of strangers and make everyone feel comfortable and at ease," he said in praise of the 33-year-old royal.
As for what he thinks of Meghan, the source told ET, "Elton thoroughly approves of Meghan and feels they are a perfect couple and very in love. Elton thinks Diana would be proud and would love Meghan."