This function returns the number of working days (Monday to Friday) between 2 dates.

function-working_days.sql

-- This function returns the number of working days (Monday to Friday) between 2 dates.
-- Tested on PostGreSQL 9.4.
-- Usage: SELECT working_days('2016-01-01'::date, '2016-01-31'::date);

CREATE OR REPLACE FUNCTION working_days(date, date) RETURNS INT AS
$$
SELECT COUNT(days)::INT
	FROM generate_series($1, $2, '1 day') AS days
	WHERE EXTRACT(DOW FROM days) NOT IN(0, 6);
$$
LANGUAGE 'sql' IMMUTABLE STRICT;