Nanne's answer given explains why you don't get the desired result (your WHERE clause removes rows), but not how to fix it.

The solution is to change WHERE to AND so that the condition is part of the join condition, not a filter applied after the join:

SELECT day.days, COUNT(*) as opens 
FROM day 
LEFT OUTER JOIN tracking
ON day.days = DAY(FROM_UNIXTIME(open_date)) 
AND tracking.open_id = 10 
GROUP BY day.days

Now all rows in the left table will be present in the result.