Seasonal transitions: The three calendars of autumn

The transition into autumn is defined by three distinct calendars: meteorological, astronomical, and Celtic. Meteorological autumn begins on September 1, covering the three months of September, October, and November. Astronomical autumn is marked by the equinox, occurring around September 22 or 23, when day and night are of equal length.

In contrast, the ancient Celtic calendar marks the beginning of autumn much earlier, on August 1, during the festival of Lúghnasadh. This period is traditionally associated with the early harvest and themes of life, death, and rebirth.

Weather patterns during this transition vary by region. In some areas, the onset of autumn is accompanied by a return of summer-like heat, with temperatures in parts of the Iberian Peninsula potentially exceeding 40°C. Other regions experience a gradual cooling, with significant temperature drops and increased moisture or frost occurring later in the season, particularly during October.