If you use a dates table, your query can be as simple as:
select dates.fulldate, count(a_ad_display.id)
from dates
left outer join a_ad_display ON dates.fulldate = a_ad_display.displaytime
[AND <other conditions here>]
where dates.fulldate BETWEEN date_add(curdate(), interval -6 day) AND curdate()
group by dates.fulldate
http://sqlfiddle.com/#!2/51e17/3
Without using a dates table, here's one way. It's ugly and probably terrible for performance:
select displaytime, sum(c)
from
(
select displaytime, count(a_ad_display.id) AS c
from a_ad_display
where displaytime BETWEEN date_add(curdate(), interval -6 day) AND curdate()
group by displaytime
union all
select curdate(), 0
union all
select date_add(curdate(), interval -1 day), 0
union all
select date_add(curdate(), interval -2 day), 0
union all
select date_add(curdate(), interval -3 day), 0
union all
select date_add(curdate(), interval -4 day), 0
union all
select date_add(curdate(), interval -5 day), 0
union all
select date_add(curdate(), interval -6 day), 0
) x
group by displaytime
order by displaytime