Hi!
Since October 6th, it is possible to book trains in France for the Christmas period (source: https://www.oui.sncf/aide/l-ouverture-des-ventes). Yet, it seems that I can’t book seat reservations for after December 11th on the Interrail site.
Anybody knows when reservations will be open? Thank you!
Usually the 60days in advance is they key rule for France.
after the 11th December is the anual Europen Timetable change and i don´t know if Eurail have to update their Reservation system by their own or if it´s automatic 
For example German Rail starts to sell Reservation on the 13th October for on the 12th December and afterwards :)
You may check on B-Europe if you can reserve the trains already (Please note they charge a service fee of 4€ instead of Interrails own 2€ and you need a PassCoverNumber) https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish
Thank you! Very detailed answer!
I will wait for a few days (a week maybe) and see if they appear. Otherwise, as you suggested, the trains are already available on B Europe.
When I tried to book on B Europe, it asked for my pass number and I used the one with 6 capital letters, but it said it was not a valid pass number. Any thoughts?
that havent a valid passcovernumber 
Passcover Numbers have a Ixxxxxx or Exxxxxx as pass covernumbers :)
with the change to the mobile version Eurail wanted to stop the PassCoverNumberSystem (and use your mobilepass code) but on few Carrier like b-europe or SJ you still need them to reserve trains. as workaround you can ask the Customer Support to get a PassCovernumber for you as workaround
https://eurail.zendesk.com/hc/en-001/requests/new
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