The European trading hours, roughly 3 a.m. to 12 p.m. Eastern, which carry the largest share of forex volume.
The four forex trading sessions. Currencies trade around the clock because the big financial centres open one after another. London and New York are both open between 12:00 and 16:00 UTC, which is the busiest stretch of the day.
London is where most institutional forex business happens. The session sets the day's range for many pairs, and its overlap with the new-york-session (8 a.m. to 12 p.m. Eastern) is the most liquid window of the day.
Traders who use session-based strategies often measure the asian-session range and trade its breakout at the London open.
Example: GBP/USD moves 20 pips during the Asian session, then 70 pips in the first two hours of London. That pattern repeats often enough that it has a name, but not reliably enough to trade blindly.