I don't know how everyone knows when they get their period.
Hi there. It sounds like you are confused about why your friends seem know when they get their periods, and why yours seems to come randomly.
Most women have what is known as a 28-day cycle. This means that every 28 days (4 weeks) they get their period. This is probably how your friends know when to expect their period.
Some women have a shorter cycle, which can be 21 days (3 weeks) and some have a longer cycle, which can be up to 40 days between periods. For more information on periods and menstrual cycles check out
You didn't say how long you have been getting your period for. When your period first starts it can take a little while for your cycle to become regular and you may experience bleeding at different times. Some women find it helpful to mark on a calendar when they get their period. That way you can see when your last period was and when your next one may be due. If you do this for a few months it might make you more familiar with your cycle, and you can see how many days/weeks there usually are between your periods.
It can be difficult when your period just comes without any warning. Sometimes there may be some signs that your period is coming. You may feel a few little pains in your stomach, you might notice yourself becoming a little moody, your breasts may start to feel sore or you may crave certain foods. It's always a good idea to carry sanitary pads and tampons around with you, so you will always be prepared no matter when your period comes.
If you are really worried about your periods you might find it useful to speak with your doctor or women's health nurse. They will be able to explain your period to you, make sure everything is functioning well and also see if there is anything they can do to make your periods more regular.
All the best.