Spring Time Table Change for European Trains

"European trains" are not a monolithic thing. Each country has their own website. For German Rail I can already see fares into October. At what country's website are you looking?

I assume you are talking about Italian regional trains. For political reasons, they don't show up on the schedule until just before the deadline. Schedule does not change much, if at all. So for travel planning purposes, just look at the train times for the same day of the week in the next week.

I have looked at Trainline.eu, Bahn, Loco2, etc. Those sites basically tell me the tickets/routes aren't available yet. I know it has something to do with the changeover on the second Saturday of June. I just wondered when the new time table would be out so I could book tickets in advance.

It will depend on the country and the type of train. In addition to Italian regional trains, some Spanish and some "eastern" European schedules are probably also not all published yet. In most cases, the summer schedule will be similar to the current schedule.

Why don't you just tell us what trips you are looking at. For Italy, you could book the high speed trains now to get SuperEconomy discounts, then book the regional connections later when they are available for sale. Regional trains are always the same price and can't sell out.