Autumn in the UK starts on different dates depending on the calendar used:

• According to the meteorological calendar, which is commonly used for weather statistics and forecasting, autumn begins on September 1 and ends on November 30.
• According to the astronomical calendar, autumn begins with the autumn equinox, which in 2025 falls on Monday, September 22, and ends on December 21.

There is also a phenological way of defining autumn by observing nature, such as leaves falling and bird migration, which can vary depending on climate and weather conditions.

In summary:

• Meteorological autumn start: September 1
• Astronomical autumn start (autumn equinox in 2025): September 22

Both are correct depending on the context in which autumn is being referred to in the UK.