There is no single answer to when autumn begins in Northern Ireland, with three calendars in use: the ancient Celtic calendar, the meteorological calendar, and the astronomical calendar.

Under the Celtic calendar, autumn began on August 1, known as Lúghnasadh. Eimear Burke, Chosen Chief of the Order of Bards, Ovates and Druids, led a gathering at a druid grove in County Kilkenny to mark the start of the season. She described it as a time of early harvest and a motif of life, death, and rebirth.

The Ulster Folk Museum follows the Celtic calendar for its events and exhibitions. History curator Fiona Byrne said the ancient Celts had a close relationship with the land, and the calendar reflected their agricultural cycles. She noted that traditions such as making harvest knots have largely disappeared, while bonfires remain common.

The meteorological calendar begins autumn on September 1. BBC NI weather presenter Barra Best said this framework allows consistent comparison of weather records across decades. He noted that the calendar is for statistical purposes and does not mean weather changes abruptly on that date.

Gary Quinn, director of rivers operations at the Department for Infrastructure, is part of Northern Ireland's civil contingencies group, which plans for weather-related emergencies. He said the focus this autumn is on potential flooding, with rivers expected to recover after a dry summer. The group maintains a state of alertness and works with Met Office colleagues.

Astronomically, autumn starts on September 23, according to Marc Sarzi, Head of Research at Armagh Observatory and Planetarium. The astronomical calendar is based on the Earth's position relative to the sun, defined by equinoxes and solstices. Sarzi noted that the constellation Orion becomes visible in late autumn and that Venus shines brightly in the evening sky.

Each calendar serves different purposes: the Celtic calendar reflects traditional land-based practices, the meteorological calendar aids climate analysis, and the astronomical calendar tracks celestial events. Byrne acknowledged the debate, saying it comes down to personal preference.