You will find that the majority of companies post during the last week and first week of the month. That isn't just a whole lot of help, but by this time of month the majority of what shows up on boards is reshops or reassignments or shops that pay too little for expectations for shoppers to take them. I would suggest each month you focus closely on several of the companies you are registered with and check their job boards several times a day to try to determine and note when they post. The next month, watch closely a few more and double check the ones you think you have figured out when they post. All of that being said, there are still companies that post throughout the month and companies that post when they get a round to it.