Spotting during times other than your period can have many different causes. It sounds like in your situation that your spotting always happens the day before you period. In this case, I think you spotting is most likely normal.
As you approach your menstrual cycle, there are a few hormonal changes that are occur. First the levels of progesterone or estrogen begin to fall. As these levels fall, the inner lining of you uterus (where menstrual blood comes from) begins to fall apart, shed, and exit the vagina. This process does not happen all at once, and can begin slowly before it reaches its maximum rate. Therefore, the spotting that your are experiencing before you period is most likely just the beginning of you period. If you experience spotting in between periods this may be do to something other than bleeding from your uterine inner lining.
I suggest that you schedule an appointment with your primary care physician
or internal medicine doctor
) or OB / GYN, whoever you see for your feminine health. He or she can take a more detailed history of your spotting and perform a thorough physical exam. If there are any abnormalities found, you doctor may do some additional testing. Good luck.