The U.S. holiday shopping season has officially begun, with shoppers crowding stores for post-Thanksgiving sales in a yearly event nicknamed “Black Friday.”

The competition for the best deals took a violent turn Thursday night, when officials say a woman at a Walmart in Los Angeles used pepper spray to gain an advantage, injuring about 20 other customers. Several shooting incidents also were reported at Walmart stores and shopping centers across the country. At least two of the incidents appeared to be robbery-related.

Black Friday is the day many stores sharply discount high-priced items such as electronics and the latest new toys. Its name signifies retailers’ expectations of high sales, as profits were once recorded in account books in black ink, while losses were recorded in red.

Stores also open their doors hours earlier than usual, which helps foster excitement among bargain hunters.

Black Friday is one of the most important days for retail chains because it indicates what they can expect during the next month of holiday shopping — their most lucrative time of year.

But not everyone is in favor of the phenomenon. Protesters with the anti-capitalist Occupy Wall Street movement were encouraging people to take part in “Buy Nothing Day,” a day of protest against consumerism.