lists all the months in a given year interval et al

list_months.sql

SELECT month_day1 :: DATE                                             AS month_starts,
       (month_day1 + INTERVAL '1 month' - INTERVAL '1 day') :: DATE   AS month_ends,
       (month_day1 + INTERVAL '1 month') :: DATE                      AS next_month_starts,
       (month_day1 + INTERVAL '1 month') :: DATE - month_day1 :: DATE AS days
FROM generate_series(DATE '2018-01-01', DATE '2018-12-01', INTERVAL '1 month') AS month_day1;