This sounds like you simply misunderstood the requirements for earning commission. From a business standpoint, this is quite a typical scenario. Many sales organizations require that a rep meet a certain % to budget before being eligible for commissions. And while I empathize with the fact that you didn't understand that the month "ended" on 5/29 instead of 5/31, it is very typical for many companies to run on different fiscal years and have billing periods that do not coincide with the first day and last day of the actual month. There is nothing you can do in this situation. You had one choice and that was to ask them to pay you, and they decided not to. I'm sure whatever documentation you signed or any job description applying to you clearly outlines the conditions under which you will and will not be paid commissions. Now that you know the details, however, you can ensure it doesn't happen again.