If you want to sort the dates column based on the date key then you can cast the column to a collection instead of an array and use sortBy on the collection
You would have to define your date/day as a model attribute first, then you can access it everywhere and also sort by this attribute:
class MyModel extends Model {
...
public function getDayAttribute()
{
//untested, just for illustration!
$dates = json_parse($this->jsonColumn);
return $dates[0]['date'];
}
}