It will be Leon's first time in a big league uniform.
"I'm very excited," said Leon, who was in the A's clubhouse Tuesday. "First time in the big leagues, it's going to be a great moment. All my family's going to be here. It's going to be a special day."
The 25-year-old's parents are flying in from Mexico for the game, and his wife and 7-month-old son will also be in attendance. His father hasn't seen him pitch in person since 2012.
Purchased by the A's from Saltillo of the Mexican League in 2008, Leon is 2-1 with a 4.34 ERA in six starts for Triple-A Sacramento this year, his seventh in the organization and third since undergoing Tommy John surgery. He combined for 24 starts at Double-A and Triple-A last season after pitching in relief in 93 of his first 105 appearances.
The A's took notice of his progress in Spring Training, when he posted a 2.13 ERA and compiled 11 strikeouts in 12 2/3 innings.
"Good stuff," said Melvin. "We've seen it the last two springs. He really seemed like this spring he looked like he belonged. He was more confident throwing to big league hitters. I told him that at the end of spring and said, 'You've put yourself in position to be a viable option for us this year,' and here he is, and well deserved."
"I was expecting this from the beginning I signed with the Oakland A's, and I didn't know it was going to take so long," said Leon. "With Tommy John surgery, I feel a lot of personal success being healthy again. I'm very proud.
"I've been feeling pretty well. All of my pitches have been there, so I'm excited to pitch here."
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.