There is no one specific day.  Different operators release the tickets at different times, so it’s just a matter of checking and checking.

One important thing - from what I have heard, the data for the rail planner app is not updated very frequently.  I think I saw that it was once a month or so.  On these timescales that’s too long for you to wait.

You should check with the national operators website as they will be the most authoritative and normally the most up to date.  Alternatively, check out bahn.de for timings as they are normally very reliable.  You can see the specific days of running of most trains on the bahn.de interface and that’s very useful.  For example it might say “runs every day 07 October to 09 December, not 10-15 November.”  This will give you an idea of the running pattern of the train, although you can’t be sure of anything (or book it) until the operator has actually released their timetable.

Just to set expectations, most operators should have their timetables up in the next few weeks.  Austria already does, for example.  However, it’s not impossible for some specific routes to do right down to the wire and not be released until the very last day (or even after).  This should only be a tiny, tiny minority of routes, but it is possible.

It is possible to make the reservation on official websites by applying the Europass? I’ll arrive in Spain on 27th December, but I don't see the trains available for those dates.