We will be in Las Vegas on Christmas Day and Boxing Day. I am interested to know if the show O will be running and if so would I need to make reservations.

You can find out for sure if it's running (rather than relying on our unofficial guesses) by going directly to their website:

www.cirquedusoleil.com

And yes, you will need to buy your tickets well in advance. The show is very popular and sells out early. Though it may take longer to sell out for Christmas Day, it almost certainly will still sell out.

Hello Sofie, I assume you are from Great Britain. We do not have Boxing Day here in the US. Have a fun time in Las Vegas.

Yeah, most Americans don't know what Boxing Day is. Anyway, you might want to contact the Bellagio and see if the tickets are on sell for Christmas Day or the 26th (see, I'm American, what can I say...but my "mum" is British so I do know what Boxing Day is) anyway, if those tickets are on sell you want to by them asap! Enjoy your stay!

Isn't Boxing Day also celebrated in Canada?

Yes, Boxing Day is celebrated in Canada. I think it is also celebrated in Australia and New Zealand.