What are the instances where a Weekly Shounen Jump serialized manga would not be released in a given week? What are the instances that there is no manga (like Naruto, Bleach and One Peace) release? Are there certain events that prevent them from releasing? Then what are these events (aside from taking a break)? Is there some sort of calendar of events for this?.
You have to understand that the manga industry is in fact an industry and normal industry situations happen.

The reasons behind a manga not coming out can be the same reasons some shop or company does not work at certain periods of time, but more specifically I've seen the following:


There is a public holiday for that period in the country of the publishing (e.g. Golden Week).
The author of the manga is taking a break after a two-chapter release. (Which basically means that the author already did this weeks job last week)
The author is on vacation.
The author is on an unpaid leave due to reasons of his concern.
The author is sick and cannot work due to that. (Pretty much calling in sick, in layman terms).


Any other reasons a normal person may not be able to work at a job all also apply here. If by chance there is a public holiday but a chapter still comes out, chances are the author prepared the chapter beforehand, and is enjoying the holiday like everyone else, although it appears that he was "working" that week.

EDIT:

As pointed out by Miharu in the comments below, there also might be a rare situation where a manga which is based on a currently airing anime has caught up to the anime and therefore might take a break to let the anime progress a little further. (Personally I haven't seen this phenomenon though.)