Each stock is listed at some exchange. There are companies that have stock listed at several exchanges, but the stock tickers have a postfix that identifies the exchange they are listed on so you can still distinguish them. And for such companies, one of those exchanges is the 'primary exchange' of the stock. Normally the vast majority of trades of a stock happen on its primary exchange.

So, since each stock has a primary exchange, the 'day' and 'night' refer to the time zone of that particular exchange.