Hi, Yes, it is okay to start taking Yaz birth control on the day of your expected period. If you start on the first day of your period, you will be protected against pregnancy immediately. If you start the pill later in your cycle, it may take up to 7 days of continuous use for the pill to become effective. Yaz birth control comes in a 28-day pack, with 24 active pills containing hormones and 4 inactive pills. It is important to take one pill at the same time each day, preferably at night, with or without food. It is also important to take the pills in the correct order and not to skip any pills. If your period comes after you start taking the pill, it may be lighter and shorter than usual, and you may experience less cramping. Some women also experience breakthrough bleeding or spotting during the first few months of taking Yaz. This is usually temporary and should improve with continued use. It is important to remember that Yaz birth control does not protect against sexually transmitted infections (STIs), so it is still important to use a barrier method of contraception such as condoms to reduce the risk of STIs. Additionally, if you have any concerns or questions about taking Yaz or any other form of birth control, it is always best to speak with your healthcare provider. If you require a personalized response, kindly use the Ask A Doctor service and post a follow-up. Take care. Hope I have answered your question. Regards, Dr. Priyanka, General & Family Physician