Checked the reset history by using the Blizz Tracker on the classic forum:
https://us.forums.blizzard.com/en/d2r/c/d2-classic-general/21After compiling and analyzing the history, here's what I came up with:
D2 LoD (classic/legacy) seasons last 6 months, with resets on the second Fridays of June and December. Servers go down at 1 PM and come up at 5 PM Pacific Time.
My projected reset date for the winter 2025-2026 season is Friday the 12th of December 2025.
My projected reset date for the summer 2026 season is Friday the 12th of June 2026.
As for when they're going to announce it, it seems to be on either the Monday or Tuesday of the week of the reset.
For the next reset, that would be Monday the 8th or Tuesday the 9th of December 2025.
For the reset after that, it would be Monday the 8th or Tuesday the 9th of June 2026.
Here's a link to the stickied official post thread for this forum:
https://forums.d2jsp.org/topic.php?t=73916272&f=21