Monday to Tuesday midnight. There's also the community rep post confirming the Monday. I also remember the Monday resets from those times. It was a long time ago and I don't mind the Tuesdays, I just came to correct the wrong.
thanks for clarification
I started around 2.3 and never knew about any other main reset besides Tuesday morning (Europe)
Everything is on Tuesday because that's the MMO industry standard. Tuesday is the MMO industry standard because companies have been gathering data for decades now and Monday night/Tuesday morning is very definitely the time when the fewest people are online in an average week. Maintenance should impact as few players a possible, so maintenance goes when the fewest people are playing, so maintenance is Monday night/Tuesday morning.
Any variation in reset times at the start of the game's life was SE imagining they'd have different needs with a worldwide playerbase, then seeing the industry standard is standard for a reason and falling in line.
It's not changing.