You need EXTRACT:
SELECT
EXTRACT(DOW FROM DATE '2011-02-16') = 0; -- 0 is Sunday
This can result in true or false, it's a sunday or it's not. I have no idea what you mean by "total number" because that will always be 0 (the date is not a sunday) or 1 (the given data is a sunday).
Edit: Something like this?
SELECT
COUNT(*)
FROM
generate_series(timestamp '2011-01-01', '2011-03-01', '1 day') AS g(mydate)
WHERE
EXTRACT(DOW FROM mydate) = 0;
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