Throw the party when ever you feel like it.
He wants to celebrate being on the promotion list, so celebrate now.
You want to celebrate the day he starts wearing the new rank, so celebrate again on that day.
It is his promotion, so let him celebrate when he wants to.
Then you can celebrate his promotion again, after he has the stripes.
Nothing wrong with two celebrations.
Heck, i celebrated making SSGT. , even though I ETS'ed the military the month before I was to be promoted.
It was the accomplishment of making the promotion list, not the promotion I was celebrating.