devtrackers.gg
It's pretty much always the Wednesday New Zealand time before the patch tho.
1) I always tell you when it is!
2) It's always on the same day respective to league launch.
3) https://i.imgur.com/jBsi5Ch.gif
4) The PatchNotesWhen™ meme is about how even though it's always the same/answered a lot, people still ask.
5) It's on the typical day before league launch. (Wednesday June 5 NZT)
6) https://i.imgur.com/pLsklhc.gif