I had hoped to purchase a train ticket from Seville to Madrid for December 26 but see that December reservations are not open yet. How far in advance are they generally available? Thank you.
Check again in a week or so...they will be up by then I believe. Might check on the site The Man In Seat 61 for additional information.
They load 60-90 days, some trains even less. If you want an idea of what will be available, check a date in September.
Since December 13 is a major date for train schedule changes across Europe, winter schedules are often published later than the typical advance window, especially in Spain, Italy, and "Eastern Europe." Schedules may be published in chunks, such as fast trains first, then regional, or sometimes vice versa.