Get first date of month in postgres

date_trunc() will do it.

SELECT date_trunc('MONTH',now())::DATE;

http://www.postgresql.org/docs/current/static/functions-datetime.html

You can use the expression date_trunc('month', current_date). Demonstrated with a SELECT statement . . .

select date_trunc('month', current_date)
2013-08-01 00:00:00-04

To remove time, cast to date.

select cast(date_trunc('month', current_date) as date)
2013-08-01

If you're certain that column should always store only the first of a month, you should also use a CHECK constraint.

create table foo (
  first_of_month date not null
  check (extract (day from first_of_month) = 1)
);

insert into foo (first_of_month) values ('2015-01-01'); --Succeeds
insert into foo (first_of_month) values ('2015-01-02'); --Fails

ERROR:  new row for relation "foo" violates check constraint "foo_first_of_month_check"
DETAIL:  Failing row contains (2015-01-02).