Booking shinkansen trips for late Dec trips

Hi I posted earlier and have decided to arrive in Osaka on 25 Dec and travel to Hiroshima on 27 Dec and then to Fukuoka on 30 Dec on JR Pass.
I am concerned about the trains being booked out.
Would it be possible to book my trips before my arrival ? If not would it be safer for us to buy Green tickets ?

Getting seat reservations on the 25th and 27th should pose no problem. The 30th is likely the most busy. But if you book right after arrival in Japan, you should still be able to make seat reservations. Basically, seat reservations for the Tokaido/Sanyo Shinkansencannot be made from outside Japan.

I too travel to japan on 19 dec arrive at osaka, on 27 dec will travel to tokyo, and on 31 dec, will travel to hakodate.
On 6 jan, will travel all the way down from hakodate to osaka. I'm going to get JR pass soon, my concerned same as Paul, do i need to book the shinkansen on coming Nov or on my arrive date.
FYI, i going to get kansai thru pass while stay in osaka, and i only start use JR pass when i travel to tokyo.

THanks.

December 31 is the only day with potential problems, especially between Tokyo and Hakodate. But if you book as early as December 19, I don't think you will have problems. Note also that the shinkansen trains between Tokyo and Aomori are some of the few trains for which there exists an online reservation website:
http://jreast-shinkansen-reservation.eki-net.com/

The other thing to understand is that 25 December(Christmas Day) in Japan is just a normal day (although this year it's on a Sunday), but 26-30th are all normal working days.