calcweekday.rb

month = 5
year = 2009

century = (year - (year % 100))/100

century_offset = ((39 - century) % 4) * 2
 
is_leap = (year % 4 == 0 && year % 100 != 0) || year % 400 == 0
 
year_offset = ((year % 100) + (year % 100)/4) % 7 
year_offset -= 1 if is_leap && [1,2].include?(month)
 
month_data = {1 => {:offset => 0, :days => 31}, 
              2 => {:offset => 3, :days => 28}, 
              3 => {:offset => 3, :days => 31}, 
              4 => {:offset => 6, :days => 30}, 
              5 => {:offset => 1, :days => 31}, 
              6 => {:offset => 4, :days => 30}, 
              7 => {:offset => 6, :days => 31}, 
              8 => {:offset => 2, :days => 31}, 
              9 => {:offset => 5, :days => 30}, 
              10 => {:offset => 0, :days => 31},
              11 => {:offset => 3, :days => 30},
              12 => {:offset => 5, :days => 31}}
 
 
days = {0 => "Sun", 
        1 => "Mon", 
        2 => "Tues", 
        3 => "Wed",
        4 => "Thr",
        5 => "Fri",
        6 => "Sat"}


# to find just the weekday, do this:
# day = 20 #calender day
# weekday_number = (century_offset + year_offset + month_data[month][:offset] + (day % 7)) % 7
# weekday = days[weekday_number]

offsets = century_offset + year_offset + month_data[month][:offset]
saturday_modulo  = 6 - (offsets)     

num_days = month_data[month][:days]
num_days += 1 if (is_leap && month == 2)

sats = []
suns = []

(1..num_days).each do |d|
   sats << d if d % 7 == saturday_modulo % 7
   suns << d if d % 7 == - offsets % 7
 end

puts "Saturdays " + sats.join(', ')
puts "Sundays " + suns.join(', ')