Doing something on Model creation

Note: I'm good with PHP and pretty OK with traditional inheritance-based OOP, but an struggling with what seems to be some pretty basic stuff in Laravel.

I have a Week model, and a Day model. Whenever I get a an instance of a Week I want to subsequently be able to do something like :

$week = new Week( <some attribtues> );
foreach (Week->days as $day) {
  <some code>
}

How can I create the days (even if they are not currently created) whenever I get a week? And how do I make sure that the days are returned as part of the Week model?

I can kinda see how I can do this using logic in a controller, but it doesn't seem to be the right place.

I can kinda see how I can create the days using events, but I don't then see how to get the days back into the Model.

This seems like a pretty basic thing to want to do: created some related objects whenever you create an object. But I'm really struggling.

Any advice?