The schema is SYSFUN.
The DAYOFWEEK function returns the day of the week in the argument as an integer value in the range 1-7, where 1 represents Sunday.
The argument must be a DATE, TIMESTAMP, or a valid character string representation of a date or timestamp that is not a CLOB. In a Unicode database, if a supplied argument is a graphic string, it is first converted to a character string before the function is executed.
The result of the function is INTEGER. The result can be null; if the argument is null, the result is the null value.