Spot Instances may not be the best mechanism to use in your case. You require the process to execute every day, and most likely to complete execution once started.
Spot instances may not run when you want them to. If your bid price is too low, or there isn't capacity, then you won't get your instance. Also, once your instance has started, if the bid price rises above your maximum price, then the instance will be terminated before it's had time to complete.
You can use a high maximum bid price to try to avoid some of these issues, but there's no guarantees.
Depending on costs, you can take a look at reserved instances (in your case, a light reserved instance may suffice) or simply an on-demand instance that you only have running at the end of your day.
Amazon does not have any scheduling of instance start/stops, so you'll need to use something like cron or other automated scheduling tools.
Amazon now has Scheduled Reserved Instances that may be usefulhelpful with regard to cost, but it won't perform the actual start for you.
