multidimensional Array or Hash with group_by 'created_at'

Hello,
i want to create a multidimensional Array or Hash with group_by
'created_at'.

3 dimensions needed:
myArray[week][day][data]

currently i only created it with 2 dimesions:

#model
def self.group_per_day
  all.group_by{|t| t.created_at.strftime("%d. %B, %Y")}.sort #"%U" for
Weeks
end

#controller
@pdf_activities = Activity.group_per_day

#view
-@pdf_activities.each do |day|
  =day[0]
  -day[1].each do |activity|
    =activity.activity

thanks