Quick code to get the most active days by hour.

activity.rb

week = Time.now.beginning_of_week - 1.week
days = [week.strftime('%Y-%m-%d')]
1.upto(6) do |i|
  days << (week + i.days).strftime('%Y-%m-%d')
end

data = days.map do |day|
  by_hour = Array.new(24) { 0 }
  Activity.
    where('date(created_at) = ?', day).
    select('hour(created_at) as hour, count(*) as count').
    group('hour(created_at)').
    order('hour(created_at)').
    map { |activity| by_hour[activity.hour] = activity.count }
  by_hour
end