We are looking at taking our family of 6 to Austria this year over Christmas. We already know dates with the wiggle of a day or two on either side that are flexible. With 6 people a few hundred bucks per person really adds up, so I am wondering when the best time would be for us to book. Especially because that is the first domino to fall for airbnb's, etc. Thanks!

Set up a Google flights alert for your preferred dates. And start watching fares. You should start to see trends.

Also, if you book Airbnbs with a generous cancellation policy, you can make changes to the reservation if you need to. That's what I do. I find foreign Airbnb hosts to be much more flexible than US. You will likely have to pay at least a 50% deposit at the time of booking, though.

FYI you can book Austrian Airlines flights using points on United or Air Canada for ~40k points each way. So if you have Chase, Amex, Capital One, etc. you could transfer points to one of the above airlines and knock a few of your tickets out with those and save some $.

This article is titled Business class but shows pricing for economy also.
https://upgradedpoints.com/travel/airlines/best-ways-to-book-austrian-airlines-business-class/

Use Booking.com for your accommodations. It's what the Europeans use…it's what I use almost 100% and I live in Switzerland. Most places have generous cancellation policies and most places you don't pay until very close to arrival date.

Lastly - I have found properties on Airbnb and then found the same property on Booking…same dates, etc…Booking has always been cheaper in my experiences.

I find international flights much cheaper mid week vs weekend. You said 5 days, I'd book flights leaving Thursday returning Tuesday. Weekend is always higher I find. I have no idea what day Christmas is on this year so the total cost of the trip might factor into your decision if the hotels are higher over the weekend.

People used to say 3 months out for international, but the last few years timing hasn't seemed to matter much. Costs in both those cabins have gone up 25% or more since COVID. I'm exclusively booking premium economy or business class so maybe the dynamics are different for economy.