The best time to post a job on craigslist is at the beginning of your posting cycle. Since chances are that you have more jobs than you can handle, starting out with a decent quantity will get the job posted and give the potential applicant some sense of your abilities before they waste their time applying for a position. A newbie would not want to apply when there are only one or two postings because it could be difficult for them to complete in such an insanely crowded market due to lack of relevant experience (and because every other person in town has posted within that timeframe).
You also should avoid posting too many jobs at once since this could result in spamming. If you put out three posts over the course of 24-48 hours, odds are very high that someone else will probably send in applications (unless they’re already planning on coming in) which means less work for yourself and potentially lower pay rates compared to someone who puts off their application until after 3 or 4 days when there’s just one available opening left by another former employer/contractor/employee.