When they get released-on/around 12/12 is the main allover EUR timings change-and if lucky some 2 month advance-if not later they will open def. timings and chance to REServe-if needed. For a rough idea on how long it takes, how the route goes etc simply use a similar weekday in sept/oct. AND be aware that 24/12, 25 and around 1/1 there may be severe reductions in the service for obvious reasons. The days just before that tend to be very, very busy.

Thanks for the reply. 

We are due to leave Berlin to return to Brussels on the 15th December, then to Paris and onto London (before the 20th). Should I have any concerns about not being able to get back, as I’m having to reserve seats so close to leaving? Sorry if I sound a little anxious. Just that we are travelling from Australia and it’s the first time we’ve utilised Eurail. Thanks 

The reservation for the Eurostar you can make already now.

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

New timetables usually appear mid-October.