@DOWI

@DOWI[mm-dd-yy]:  Returns the day of the week for the specified date as an 
integer (1 = Sunday, 2 = Monday, etc.).  DATE uses the date format and 
separators mandated by your country code (for example dd.mm.yy in Germany, 
or yy-mm-dd in Japan).  The year can be entered as a 4-digit or 2-digit 
value.  Two-digit years between 80 and 99 are interpreted as 1980 - 1999; 
values between 00 and 79 are interpreted as 2000 - 2079.