Unlike traditional markets, the markets that trade in bitcoin never close, so the price of a bitcoin relative to fiat currencies (like the US dollar) may change from second to second, 24 hours a day, 7 days a week.
Bitcoin doesn’t take holidays off, either.
The price of bitcoin is determined by the price that people are willing to trade it for. Supply (Bitcoin has a limited supply) and demand (More people wanting more bitcoin).
You don’t have to buy a whole bitcoin - you can buy a fraction of a bitcoin, and secure it yourself.