You should be able to withdraw RMB from an ATM easily if you have the proper type card. Make sure that your card has a Visa or Mastercard logo and you'll be fine.

I don't know if there are ATMs located on the airside in Beijing. There will be plenty once you exit customs in Guangzhou. There might also be some in baggage claim.

And the previous poster is correct, most Chinese banks are open on the weekend. ATMs may be out of cash on Monday morning so plan accordingly.