My Mom was diagnosed shortly before Thanksgiving as well... I remember so well the looming feeling of the holidays coming and... the pain and the
"--you want me to be freaking happy?!" feelings that I had on top of wading through those first days of terror.
First of all, you've gotten so much great advice so far. Consider a second opinion. Support your Dad no matter what, but do let him know that there IS HOPE!!!!
The second thing I have to say is something I learned these last two holidays first hand--Holidays don't have to be all about "happiness." I'm not sure what faith you may practice, and I'm not preaching, but... I am a Christian--and I realized hardcore these last two years that the very first Christmas was about a light piercing darkness. I think a lot of the other holiday faiths celebrate similar things when you get right down to it.
Once I grasped the idea of it being about light piercing darkness, I looked desperately for that light anywhere I could find it. And find it I did... Last year's Christmas was especially bleak for me--but... I still found that light. I didn't make myself feel happy, but I tried to see that light any chance I got... I let myself cry too. Because... The darkness is as real as the light.
Anyway--All that to say, I know it is hard, but grab onto hope, grab onto that light all that you can. For the holidays and for this entire journey.
And we'll help you to keep doing that.
Val