I know this sounds like a stupid question but no one ever explained it in health class

None of the above. They have it at regular or sometimes irregular cycles. They have them any day of the month, Each women may be different.

It can begin at any time during the month. Each woman is different. Often, each month varies for each woman. The birth control pill will regulate a woman's period, but stress, illness, forgetting a pill, and even the weather can mess up the schedule.

It can be at the beginning, middle, end, or anywhere in between. Every woman has a different cycle.

It is called a monthly cycle because the average length of a woman's cycle is a month. It can be longer then a month, sometimes two or three depending on the person medical state, or it can be shorter then a month. The AVERAGE is about 28-30 ish days.

The Calendar as it stands is based upon when women start their cycle. It would be convenient for some people, but it was never the intent. That being said, it has been observed that when a group of women live together or are together for an extended period their cycles will tend to line up and synchronise.

Since, it's a monthly cycle it can begin at any time or any date in a month. So, your thought is actually wrong. Because this process is basically continued after every 28-30 days.