My bitcoin wallet "US dollar" amount is changing. It's Saturday the market is closed. How is this happening?
To my knowledge, bitcoin is not traded on any stock markets. It is traded directly on exchanges specialized in cryptocurrencies. These exchanges don't necessarily close, and therefore trades are always occurring.
Your wallet is reading the price feed from one of these exchanges (you can probably configure which one if you look at the settings), and is therefore getting updates about the going rate on that exchange. Whether a stock market is open or closed it's irrelevant.