The WEEKDAY() function returns the weekday index of the week of "Date" in the range 0 to 6.
0 = Monday, 1 = Tuesday, 6 = Sunday.
The "Date" argument must be of type DATE or DATETIME.
For invalid dates like 0000-00-00 that have a 00 day part, the function returns NULL.
If the value for "Date" is invalid or NULL, the WEEKDAY() function returns NULL.