A general rule of thumb is that Off-Peak will begin at 09:30 Mondays to Fridays in cities and large towns, and 09:00 across the rest of the network. Weekends and Bank Holidays are Off-Peak all day.
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What is off-peak?
Generally speaking, the Off-Peak window begins after 09:30 and outside of evening commuter hours - 15:30-18:15 - for travel in and out of big cities. Super Off-Peak tickets become available after 10:00.