Ovulation occurs before a woman actually menstruates, that is, the "period" part of the menstrual cycle. They are two seperate processes, as ovulation is the actual release of a follicle/egg cell. Menstruation occurs when the cell is not fertilized, and is then expelled from the body, along with the uterine lining which has thickened, should fertilization occur.
There is no "common place" way of knowing when you are ovulating- it is different for every woman, and is not something that you can feel, or really test for.
What exactly do you mean by you have never had a "regular" period? You mention that your period occurs at roughly the same time- this is normal. If you have other menstrual irregularities, I suggest trying to get in touch with a good gynaecologist, that is a physician who specializes in female health. Usually, the menstrual cycle regulates itself by the time a woman is your age; so if there are other concerns, you might want to seek medical attention
Most periods usually last around 3-7 days.
Engaging in sexual activity at any time, regardless of when this takes place in relation to menstruation always carries the risk of pregnancy. Even if such activities are "protected", there is always a chance. For the sake of anwering your question, I will tell you that yes, there are times during a woman's menstrual cycle when she is less likely to become pregnant, but I do not encourage you to take this chance. I don't mean to judge you, or read too between the lines of what might be a simple question on your part, but since this is the Internet, and this information is accessible to the public, I just feel the necessity to emphasize that you shouldn't take such chances