During the months leading up and during September, there is often a larger amount of people looking for employment. With many families having children that are about to start school in September, parents are often no longer needed at home so much and therefore often need to get a full or part time job. This is also a time when students are starting to look for jobs working up towards Christmas, so you may find that you are also competing against many of them too.
If you are looking to return to employment during September then I would strongly recommend start to look and apply as soon as you can. It may be that you need to start the job early, but if you might be able to get friends and family to help out for a few weeks or so, then at least you can secure a position before the rush starts.
Starting the job before September will also mean you have had chance to settle in prior to your children starting school and may make it easier should you have to have any time off in the first week or so when your child is settling in to classroom life.