Deleting rows at the end of each month run - sql

Question

I am executing my test_function as below
select test_function(t.dt::date)
from generate_series(date '2018-01-01', date '2019-12-01', interval '1 month') as t(dt);
Definition of test_function
CREATE OR REPLACE FUNCTION test_function(_snapdt date)
RETURNS integer
AS
$BODY$
declare rows integer;
BEGIN
DROP TABLE IF EXISTS temp_table;
create table temp_table AS
select col1,col2
from table1
where id=01000 and
snapshot_date=_snapdt
group by col1,col2
distributed randomly;
INSERT INTO standard_table
( snap_date,
col1,
col2
)
SELECT
_snapdt as snap_date,
a.col1,
a.col2,
FROM temp_table a;
GET DIAGNOSTICS rows=ROW_COUNT;
DROP TABLE IF EXISTS temp_table;
return rows;
END;
$BODY$
LANGUAGE plpgsql;
It executes for each month from 2018 to 2019.
I am curious how do we delete records based on a specific condition at the end of each month run.
ex: after completing executing 2018-01-01 it should run below statement
delete from my_table where col4 is NULL;
Same ,after completing executing 2018-02-01 it should run below statement
delete from my_table where col4 is NULL;
and so on for all the months from 2018 to 2019 (statement is same for all the months)