select * from days where date < '2018-10-09' order by date desc limit 1

fails if there is an index on date and a record for 2018-10-09 does not exist in the table.

To recreate ....

create table days (date date);
insert into days values ('2018-10-04'), ('2018-10-05');
select * from days where date < '2018-10-09' order by date desc limit 1; # Works as expected
alter table days add index date_index (date);
select * from days where date < '2018-10-05' order by date desc limit 1; # Works as expected
select * from days where date < '2018-10-09' order by date desc limit 1; # FAILS: Empty set (0.001 sec)