August marks the beginning of autumn, according to the Celtic calendar 

Met Éireann says it uses 'meteorological seasons to divide the year into four seasons of equal length [three months each]' as it 'helps enormously in the calculation and provisional of seasonal statistics'. Picture: Larry Cummins

While most believe summer ends in August, the Celtic calendar shows the season ending in July, therefore making Tuesday the 'traditional' beginning of autumn.

According to Met Éireann, the Celtic calendar — with seasons Imbolc (spring), Bealtaine (summer), Lughnassa (autumn) and Samhain (winter) — differs from the meteorological calendar which is used by the forecaster.

Meteorological seasons are based on the annual cycle of temperatures rather than the position of the Earth compared to the sun.

The practice of using seasons as a means of explaining weather patterns has dated back as far as 1780.

Met Éireann says meteorological seasons are simpler and easier to remember than astronomical seasons, for example, as the start and end dates for astronomical seasons vary each year.

	Traditional/Celtic	Astronomical	Meteorological
Spring (Imbolc)	1/2 Feb St Brigid’s Day	20 March Vernal Equinox	March, April and May
Summer (Bealtaine)	1 May	20/21 June Summer Solstice	June, July and August
Autumn (Lughnasa)	1 August	22/23 September Autumnal Equinox	September, October and November
Winter (Samhain)	31 Oct/ Nov	21/22 December Winter Solstice	December (year), January and February (year after)

Met Éireann says it uses "meteorological seasons to divide the year into four seasons of equal length [three months each]" as it "helps enormously in the calculation and provisional of seasonal statistics”.

The forecaster also implements a separate “storm season”, beginning from September 1 each year for the storm naming project.

On the difference between the Celtic and meteorological calendar, Alan O’Reilly of Carlow Weather said: “The main difference in simple terms is August is a warmer month than May and from a weather point of view it is summer so the meteorological season follows weather.”

Mr O’Reilly said the Celtic season is based on the amount of light combined with “daylight hours”, with the astronomical season following the solstice and not calendar months.

Some think summer ends tonight… pic.twitter.com/DjuhKgxxrG

— Carlow Weather (@CarlowWeather) July 31, 2023

Despite the differences in the various calendars, Mr O’Reilly said it was less a way of predicting the weather and more focused on "what people see as the seasons", adding  the calendar everyone chooses to follow is "a point of view you choose to believe".

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