I'm about to book a flight to Paris and want to buy coach and use miles to upgrade to biz.
As of now, I'm told there are no upgrade seats available but I can buy my coach tkt and go on the upgrade list. I'm first tier elite.

As I look at the Biz Class seats available on my flight options, I see that all of them still have at least 20 empty biz class seats. I'm flying the 1st week of Dec. - so not exactly the most popular time to visit Paris.

My question is: Which flight should I book to give me the best chance? I can fly on Fri, Sat or Sun and there are flights at 6:00PM and 9:00PM.

Any hints as to which are most/least popular times to fly from the US to Europe?

Thanks.