I have not seen a specific thread nor forum area where a new ladder season date is posted ahead of time. What generally happens is the day of when the new season is to begin, a mod or VIP may post a heads-up about when the servers go down and how long they may go down for during the ladder reset. It is a bit hard to plan around a ladder reset.
What we used to do with ladder reset teams is guess the closest Tuesday to 6 months and a week before they would put info on login page or a blue would make a post in old D2 classic forum telling us.
It is an archaic system that needs updating.
I wonder if D2 2000 and D2R will have same season start next year when D2R releases.
It would be neat if they coincided like that, although if they go with 3 month seasons like was discussed, I guess they would get out of sync pretty quickly.