How to get the last day of month in postgres?

For anybody coming to this question looking for the Postgres way to do this (not using Redshift), here's how you'd do it:

SELECT (date_trunc('month', '2017-01-05'::date) + interval '1 month' - interval '1 day')::date
AS end_of_month;

Replacing the '2017-01-05' with whatever date you want to use. You can make this into a function like this:

create function end_of_month(date)
returns date as
$$
select (date_trunc('month', $1) + interval '1 month' - interval '1 day')::date;
$$ language 'sql'
immutable strict;

EDIT Postgres 11+

Pulling this out of the comments from @Gabriel, you can now combine interval expressions in one interval (which makes things a little shorter):

select (date_trunc('month', now()) + interval '1 month - 1 day')::date as end_of_month;

-- +--------------+
-- | end_of_month |
-- +--------------+
-- | 2021-11-30   |
-- +--------------+
-- (1 row)

Okay, so you've got a numeric(18) column containing numbers like 20150118. You can convert that to a date like:

to_date(your_date_column::text, 'YYYYMMDD')

From a date, you can grab the last day of the month like:

(date_trunc('month', your_date_column) + 
    interval '1 month' - interval '1 day')::date;

Combined, you'd get:

select  (date_trunc('month', to_date(act_dt::text, 'YYYYMMDD')) + 
           interval '1 month' - interval '1 day')::date
from    YourTable;

Example at SQL Fiddle.

date_trunc('month',current_date) + interval '1 month' - interval '1 day'

Truncating any date or timestamp to the month level will give you the first of the month containing that date. Adding a month gives you the first of the following month. Then, removing a day will give you the date of the last day of the month of the provided date.

If you're using Amazon AWS Redshift then you can use Redshift's LAST_DAY function. While Redshift is based on PostgreSQL, the LAST_DAY function is not available in PostgreSQL, for a solution for PostgreSQL see @wspurgin's answer.

https://docs.aws.amazon.com/redshift/latest/dg/r_LAST_DAY.html

LAST_DAY( { date | timestamp } )

LAST_DAY returns the date of the last day of the month that contains date. The return type is always DATE, regardless of the data type of the date argument.

For example:

SELECT LAST_DAY( TO_DATE( act_date, 'YYYYMMDD' ) )