It depends why it has been withheld but assuming there is no specific reason (such as you breaking something they are insisting you py for), they are obliged to pay wages as due. The 15th is a Sunday so most companies would put it in your account on the Friday (13th) but should be in your account by midnight on Sunday night/Monday morning at the latest.
Payment terms should be stipulated in your contract or written statement of particulars. If you don't have a contract, then consider how they normally pay. If they normally pay on the 15th then that would probably stand. However, if overtime is paid on an as-and-when basis rather than peridoically, you probably won't get far.
However, there may be other circumstances such as overdue payments that they are owed which may be why they are with holding. Perhaps you should go to whoever is in charge of the payroll and ask why it is being with-held. Explain your situation and if they are reasonable people, they may put it through anyway. I'm sure they wouldn't want to risk having disgruntled workers unless the absolutely had to.