This Thursday is May 1st, which is a holiday in Europe and Asia, but not in the US. Whenever there is a holiday in any of the trading regions (US, Europe, or Asia), markets throughout the world are affected (even those that are still open).

European and Asian markets will be closed on Thursday, and even though the US markets will be open, the missing European and Asian traders will cause the US markets to trade differently than usual.

Friday is not a holiday, and the European and Asian markets will be open, but many traders will take fours day off (Thursday through Sunday), so trading on Friday may be lighter than usual (i.e. may have lower volume).

Whether the holiday affects your trading depends upon the markets that you trade, your trading style, and your trading system. If you do not yet know how holidays affect your trading, just be aware that you may need to make some adjustments to your trading to compensate.