El CalorZIHto Que Viene

I'm curious about La Canícula... is it the same dates every year based on historical tradition, or is it something measurable that signals the start (like El Nino and water temps)? Is it specific to Zihua, or is it a thing all up and down the coast?

I've heard people talk about different times of the summer having better/worse conditions for various activities (diving, fishing, etc) and trying to piece it all together into better knowledge about how things work here. Thanks!