Direct answer
The London forex session is commonly considered to start at the time London financial markets open for the trading day (UK local time). In practice, different platforms may mark a “session start” based on liquidity or their own trading-hours definition, so the exact minute can vary.
Explanation: what “start” means
A “forex session” usually describes when a region’s largest banks and liquidity providers are active, which affects trading volume and spreads. For the London session, the most stable reference point is the London market opening time in the UK time zone.
Because the UK observes daylight saving time, the session start time is often discussed in two ways:
- UK winter time (standard time)
- UK summer time (daylight saving time)
Without a specific date, you cannot convert these references into one fixed clock time for everyone. Instead, treat the London session start as “London open time in UK local time,” then adjust to your local time zone using daylight-saving rules.
Also note that “start” can be defined in more than one way:
- Market-hours start: when London markets open (the most common conceptual definition).
- Trading-platform session start: some brokers or data providers begin labeling the session slightly before or after the cash market open, based on their feed or liquidity window.
Example checks you can do
To pin down the time for your exact use case, check the following independent items:
- Your data source’s session table: many platforms label session windows (including London) on their website or in market-data settings.
- Your calendar date and time zone: confirm whether the date you care about is under UK summer time or UK winter time.
- Holiday closures: if London markets are closed or have reduced hours, the effective “session start” window may shift.
- Compare two time references: one from a market-hours calendar and one from your platform’s session definition. If they differ by minutes, use the platform definition for consistent analytics.
Limitations and uncertainty
- No single universal minute: “London session start” can differ by definition (market open vs. liquidity label) and by broker/data provider.
- Not real-time: session labeling does not update from your personal circumstances; it follows standard calendars and provider conventions.
- No guarantee of liquidity: even when the session is “open,” spreads and activity can change due to news, holidays, and provider-specific routing.
For the most accurate answer on a specific date, use a current market-hours calendar for London and, if available, your provider’s published session-window definition.