What is open on Christmas day in Oaxaca
 

I am in the early stage of planning a trip to Mexico in December. We plan to spend Christmas in Oaxaca, as there are many festivals on days leading to Christmas. But what can we do on Christmas day itself? I assume markets and museums will be closed. What about Monte Alban? Is it open? If so, can we easily find transportation to/from the ruin?

Alternatively, we could get to Oaxaca a day or two earlier to see the sights, and take the bus to DF on Christmas day. Will bus service be reduced in holiday?

well, If you have stayed up most of the night with the locals, going to church or partying you will be sleeping. I think you are right to travel that day. I'm sure the bus schedule will work - buy your tickets ahead of time or at least check the schedule
http://www.ticketbus.com.mx/wtbkd/index.jsp
There will be a few city buses running, some taxis and the restaurants in the zocolo. In the late afternoon a few little food places will open. Be quiet - we're all sleeping!
Have fun!

Pretty much everything is open in the afternoon though probably not Monte Alban. If you have a moment you could read my travel blog- just click on 2011.
http://andrewchum.blogspot.co.uk/
I was there last Christmas and New Year- spent four weeks learning Spanish.

Thanks, we will plan to be on the bus on Christmas day.