給日期，問那天星期幾

Doom’s day algorithm is not a method to compute which day the world will end. It is an algorithm created by the mathematician John Horton Conway, to calculate which day of the week (Monday, Tuesday, etc.) corresponds to a certain date.
This algorithm is based in the idea of the doomsday, a certain day of the week which always occurs in the same dates. For example, 4/4 (the 4th of April), 6/6 (the 6th of June), 8/8 (the 8th of August), 10/10 (the 10th of October) and 12/12 (the 12th of December) are dates which always occur in doomsday. All years have their own doomsday.
In year 2011, doomsday is Monday. So all of 4/4, 6/6, 8/8, 10/10 and 12/12 are Mondays. Using that information, you can easily compute any other date. For example, the 13th of December 2011 will be Tuesday, the 14th of December 2011 will be Wednesday, etc.
Other days which occur on doomsday are 5/9, 9/5, 7/11 and 11/7. Also, in leap years, we have the following doomsdays: 1/11 (the 11th of January) and 2/22 (the 22nd of Febrary), and in non-leap years 1/10 and 2/21.
Given a date of year 2011, you have to compute which day of the week it occurs.