Trading sessions
The Asian (Tokyo), London and New York sessions; the London–New York overlap (around 15:30–19:00 Turkey time, UTC+3) is the most liquid window.
Forex is open 24 hours, but its character changes by session: the Asian session means a tight range, the London open brings direction and volume, and the New York open (16:30 Turkey time, the stock market open) brings volatility. When daylight saving time changes, the Turkey time equivalents shift by one hour.
Many strategies only work in certain session windows; adding a session filter to a backtest changes the result noticeably.