Something

SELECT MAX(days.created_at),
(SELECT value FROM readings AS r0 
 WHERE r0.created_at < MAX(days.created_at)
 ORDER BY r0.created_at desc LIMIT 1) as foo
FROM readings AS days 
WHERE days.meter_id = 1 
GROUP BY YEAR(days.created_at), 
         MONTH(days.created_at), 
         DAYOFMONTH(days.created_at)