Having cleared his absence with coach Brendon McCullum, he will not take part in England's session that morning. As a result, he will only have trained with the squad twice before the start of the series.
"I've been trying to book it for a while, but I've not really had a date," Moeen said. "I got this date after quite some time. I spoke to Baz, and he said it's fine.
"I bought a suit, a three-piece suit. It's going to be boiling. I had to buy a new one. My mum and dad love it. I'm taking them, and my missus. We're going to enjoy the day."
His decision to reverse his retirement from Test cricket and make himself available for the Ashes prompted a substantial change of plans. Moeen had expected to spend June and July captaining Warwickshire in the T20 Blast, and admitted that his wife, Firoza, "wasn't that keen" when he first floated the idea with her.
"I did have some family holidays booked," he said. "There weren't many. [We were going to] the Cotswolds, nowhere big. It was more of a family get together with my in-laws… nothing major. My wife wasn't that keen, but everyone else around me was.
"At the back end of my [Test] career, she knew that I didn't enjoy it as much, and that I was much happier with the way things were going and enjoying my cricket [over the last two years]. So that took a lot of convincing.
"That last thing I would want to do was say 'yes' and not be in it and properly with everything. One thing I really want to make clear is that I'm going to try to give this everything I've got - more than I did before, probably - and be a massive part of what could be a huge series. I want to be involved in this."
Moeen admitted that the fact that the first Test will be held in Birmingham, where he lives, "made a bit of a difference" in his decision-making. "It wasn't a massive part, but it was helpful because I only live around the corner and I still get to see my family in the evenings," he said.
He also clarified that his comeback was unrelated to Rishi Sunak, the UK's Prime Minister, calling for England to "send an SOS to Moeen Ali". Moeen said: "That didn't convince me… I'd really made my mind up before that."