|Day of week|.coffee

day_of_week = (day, month, year) ->
	month_len = [0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]	# 0 ? Because jan.
	month_len[2] += 1 unless year % 4 or year % 1000 == 0		# 29th february, you know.
	unless 0 < month <= 12						# Month boundary check.
		throw new RangeError("Month index outside of [1..12] range !")  
	unless 0 < day <= month_len[month]				# Day index boundary check.
		throw new RangeError("Day index outside of [1..#{month_len[month]}] range !")
	#.---Finally some actual calculations:
	dayz = day +							# Amount of days passed.
			month_len[...month].reduce((a, b) -> a + b) +	# Sum(days_in_jan..prev_month) + 1
			--year * 365 + year // 4 - year // 1000		# Leap years correction.
	return dayz %% 7

date2day = (args...) ->
	['mon', 'tue', 'wed', 'thu', 'fri', 'sat', 'sun'][day_of_week args...]

## Example...
console.log date2day(3, 12, 2014) # => 'wed'