In the real world...
...lots of these breaches are not very big and not very exciting. Think documents left on the bus rather than cyber attackers.
Thought process if the breach occurs on a Monday - Lets find out what happened and wait until the end of the deadline to make sure we have all the facts.
Thought process if the breach occurs on a Thursday - Do I really want to spend my weekend sorting this out or shall I just chuck the notification on Friday afternoon...?