Forex sessions and where they overlap
The London and New York overlap runs 08:00 to 12:00 New York time, which is 13:00 to 17:00 in London. Four hours, and the deepest liquidity of the day. For about four weeks a year it is five, because the two sides change their clocks on different dates.
Before the table: these are conventions, not opening hours. Forex has no exchange, no bell and nothing to publish a schedule. Nobody switches Tokyo off at six. What the four session names describe is when the dealing desks in each centre are at work, and the convention is worth knowing precisely because everybody else is using it: it is what the sessions on your platform are drawn from and what the commentary means.
The four centres
Opens the week. Thin, and the place a Monday gap shows up first.
The Asian session. Yen crosses are at their most active here.
The largest by volume, by a distance. The 16:00 London fix is the benchmark most institutional orders are priced against.
US data lands at 08:30 ET, which is why the first hour is the busiest of the day.
Your own column is worked out in your browser from your device clock, so it is right wherever you are reading this and it follows daylight saving on both ends. That is the part a static table cannot do, and daylight saving is where these get confusing: for three weeks each spring the US has switched and Europe has not, and the overlap moves an hour without anybody announcing it.
The overlaps, in order of how much they matter
London and New York deepest
Both of the two largest centres are at their desks. Spreads are at their tightest, the largest orders go through, and the day's range is usually made here. If there is one window that matters, this is it.
Tokyo and London brief
One hour while London is on GMT and two while it is on summer time, because Tokyo does not change its clocks and London does. Mostly one-sided either way: Tokyo is winding down as London arrives. It matters for yen crosses and for very little else.
Sydney and Tokyo quiet
The quietest overlap of the three. Ranges are narrow, and a move here usually means something happened while the West was asleep.
What this does not mean
It does not mean the market is shut outside these hours. It runs from Sunday 17:00 to Friday 17:00 New York time without stopping, and a price is available at three in the morning in every one of these centres. What changes is how much is behind that price.
It does not mean the overlap is the right time to trade, either. It is the window with the tightest spreads and the most volume, which also makes it the window where the largest moves happen. Those are the same fact described twice, and which of them matters to you is not something a calendar can tell you.
And it does not mean much on a public holiday. The market stays open when London or New York is shut, and the liquidity does not: those are the days the spread on a major pair quietly doubles. Those dates are on the forex page.
Where these hours come from
Not from an exchange, because there is not one. They are the hours the industry quotes for each centre's dealing day, and they are stated here in each centre's own local time so that daylight saving is handled by the conversion rather than baked into a number that goes wrong twice a year. Different brokers draw the boxes slightly differently, by an hour at either end. Nobody is wrong, because there is nothing to be wrong about: it is a description of when people are at their desks, not a rule anybody enforces.
The weekend close is different. That one is real, it is the same everywhere, and it is on the forex page with the thin-liquidity dates.