SELECT MAX(adate) FROM atable WHERE adate BETWEEN (('\''||
EXTRACT(YEAR FROM dateparam) || '-' ||
 EXTRACT(MONTH FROM dateparam) ||'- 01 \'')::timestamp) 
AND
 (('\''||
EXTRACT(YEAR FROM dateparam) || '-' ||
 (EXTRACT(MONTH FROM dateparam)+1) ||'- 01 \'')::timestamp)

SELECT * FROM atable AS a
WHERE adate =
(SELECT MAX(adate) FROM atable WHERE adate BETWEEN (('\''||
EXTRACT(YEAR FROM a.adate) || '-' ||
 EXTRACT(MONTH FROM a.adate) ||'- 01 \'')::timestamp) 
AND
 (('\''||
EXTRACT(YEAR FROM a.adate) || '-' ||
 (EXTRACT(MONTH FROM a.adate)+1) ||'- 01 \'')::timestamp))

SELECT a.* FROM atable a INNER JOIN (
SELECT MAX(adate) as madate FROM atable
GROUP BY 
EXTRACT(YEAR FROM adate),
EXTRACT(MONTH FROM adate)) q ON a.adate=q.madate