Day of the week at a given date

This script calculates the day of the week for any valid 
date in years 1901-2038 in the Gregorian calendar. 
The date format is the format defined in ISO 8601, where
the largest temporal term (the year) appears first and the
smallest term (day) appears last. The separator of the terms is
hyphen (-). The day and month must be given as 2 digit numbers,
and the year as a 4 digit number. Examples of possible dates:
2010-02-28, 1944-07-05 

Give the date (eg 2018-01-20):

Day of the week at 2018-01-20 = Saturday

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