Sunday is 1 (1 or omitted): Sunday is the first day (day 1) of the week and Saturday is day 7.
Monday is 1 (2): Monday is the first day (day 1) of the week and Sunday is day 7.
Monday is 0 (3): Monday is the first day (day 0) of the week and Sunday is day 6.
Examples
=WEEKDAY("Apr 6, 1988",1) returns 4 (Wednesday, the fourth day if you start counting from Sunday as day 1).
=WEEKDAY("Apr 6, 1988") returns 4, the same value as the preceding example (numbering scheme 1 is used if no number-scheme argument is specified).
=WEEKDAY("Apr 6, 1988",2) returns 3 (Wednesday, the third day if you start counting from Monday as day 1).
=WEEKDAY("Apr 6, 1988",3) returns 2 (Wednesday, if you start counting from Monday as day 0).