-- NOTE THAT THIS IS JUST *ONE* POSSIBLE SOLUTION TO THE QUESTION

-- type declarations

type Day = Int
type Month = Int
type Year = Int
type Date = (Day,Month,Year)

-- basic functions

dpm :: Year -> [Int]
dpm year = [31,feb,31,30,31,30,31,31,30,31,30,31]
	where feb
		| (leap year)	= 29
		| otherwise	= 28

dayName :: Int -> String
dayName day
	| day==0	= "monday"
	| day==1	= "tuesday"
	| day==2	= "wednesday"
	| day==3	= "thursday"
	| day==4	= "friday"
	| day==5	= "saturday"
	| day==6	= "sunday"

leap :: Year -> Bool
leap year 
	| (year `mod` 100)==0 && (year `mod` 400)==0	= True
	| (year `mod` 100)==0 && (year `mod` 400)>0	= False
	| (year `mod` 4)==0				= True
	| otherwise					= False

-- functions for dates after 01/01/2000

numDaysSince2000 :: Date -> Int
numDaysSince2000 (day,month,year)
	=   day					-- days this month
	  + sum (take (month-1) (dpm year))	-- days this year
	  + (year-2000) * 365  + length [yr | yr <- [2000..year-1], leap yr]  -- days this century

dayOfWeekPost2000 :: Date -> String
dayOfWeekPost2000 date 
	| (numDaysSince2000 date) == 1		= dayName 5
	| (numDaysSince2000 date) == 2		= dayName 6
	| (numDaysSince2000 date) == 3		= dayName 0
	| (numDaysSince2000 date) == 4		= dayName 1
	| (numDaysSince2000 date) == 5		= dayName 2
	| (numDaysSince2000 date) == 6		= dayName 3
	| ((numDaysSince2000 date) `mod` 7)==0	= dayName 4
	| ((numDaysSince2000 date) `mod` 7)==1	= dayName 5
	| ((numDaysSince2000 date) `mod` 7)==2	= dayName 6
	| ((numDaysSince2000 date) `mod` 7)==3	= dayName 0
	| ((numDaysSince2000 date) `mod` 7)==4	= dayName 1
	| ((numDaysSince2000 date) `mod` 7)==5	= dayName 2
	| ((numDaysSince2000 date) `mod` 7)==6	= dayName 3
	| otherwise				= "???"

-- functions for dates prior to 01/01/2000

numDaysBefore2000 :: Date -> Int
numDaysBefore2000 (day,month,year)
	=   (((dpm year) !! (month-1)) - (day-1))				-- days left in this month
	   + sum (take (12-month) (reverse (dpm year)))				-- days left in this year
	   + (1999-year) * 365  + length [yr | yr <- [year+1..1999], leap yr]	-- days left in years up to 2000

dayOfWeekPrior2000 :: Date -> String
dayOfWeekPrior2000 date 
	| (numDaysBefore2000 date) == 1		= dayName 4
	| (numDaysBefore2000 date) == 2		= dayName 3
	| (numDaysBefore2000 date) == 3		= dayName 2
	| (numDaysBefore2000 date) == 4		= dayName 1
	| (numDaysBefore2000 date) == 5		= dayName 0
	| (numDaysBefore2000 date) == 6		= dayName 6
	| ((numDaysBefore2000 date) `mod` 7)==0	= dayName 5
	| ((numDaysBefore2000 date) `mod` 7)==1	= dayName 4
	| ((numDaysBefore2000 date) `mod` 7)==2	= dayName 3
	| ((numDaysBefore2000 date) `mod` 7)==3	= dayName 2
	| ((numDaysBefore2000 date) `mod` 7)==4	= dayName 1
	| ((numDaysBefore2000 date) `mod` 7)==5	= dayName 0
	| ((numDaysBefore2000 date) `mod` 7)==6	= dayName 6
	| otherwise				= "???"

-- top-level function

whatDay :: Date -> String
whatDay (day,mth,yr)
	| yr < 2000	= dayOfWeekPrior2000 (day,mth,yr)
	| otherwise	= dayOfWeekPost2000 (day,mth,yr)