My solution to EPP13 puzzle #7 - jfmamjJASONd. Puzzle available at: http://www.ericharshbarger.org/epp/2013/puz/07_jfmamjJASONd/jfmamjJASONd_v04.pdf

jason.rb

require 'colorize'
require 'csv'

def color? date
  date.wday == 5 # Friday
end

CSV.foreach('grid') do |row|
  row.each do |cell|
    next unless cell
    month, year = cell.split ','
    year  = "0#{year}" if year.to_i < 10
    month = "0#{month}" if month.to_i < 10
    pref  = year.to_i <= 13 ? 20 : 19
    formatted = "#{pref}#{year} #{month} 13"
    date = Date.strptime formatted, '%Y %m %d'
    if color? date
      print " ".on_white
    else
      print " "
    end
  end
  puts
end