code.rb

dates.each_slice(7).with_index.with_object({date:0, visits: 0, rate: 0 })   do |(*arr, i), accumulator| 
      accumulator[:date] = arr[0][0].to_i
      accumulator[:visits] += arr[0][1].to_i
      accumulator[:rate] += arr[0][2].to_i
      accumulator[:rate] / 7 if  i == 7
    end

data.rb

[
['01','313', '33.60'],
['02','312', '13.50'],

['03','323', '31.40'],
['04','123', '33.30'],

['05','232', '31.10'],
['06','311', '23.60'],
]

# => [6, 1614,166.5 ]
#[last, sum, average]