Icelandic Calendar: Why Thursday for first day of Summer?

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Icelandic Calendar: Why Thursday for first day of Summer?

Karl Palmen - UKRI STFC

Dear Calendar People


I have wondered why the Icelandic Calendar begins the summer season with a Thursday.


According to "Mapping Time" by E. G. Richards, the Icelandic calendar was a leap week calendar. This year was divided into two halves. A dark winter half of six months of 30 days and a light summer half of six months of 30 days plus 4 epagomenal days. A leap week was occasionally added to the middle of summer, eventually to ensure that the summer half began on "a" Thursday between 9 and 15 April in the Julian calendar. Firstly, the "a" can be replaced by "the", because such a Thursday would be unique and thus the leap week years determined.


"Mapping Time" does not mention when the epagomenal days occurred other than they are within the summer half. A convenient time to add the leap week would be the epagomenal days, because this would not add any interruptions to the 30-day month cycle. This suggests that the epagomenal days were in the middle of summer between the 3rd and 4th months of summer.


Now I work the day of week each month starts in backward steps of 3 months = 90 days = 13 weeks - 1 day and get.


1st month Summer Thursday (9-15 April)

4th month Winter Friday

1st month Winter Saturday

4th month Summer SUNDAY


So if the epagomenal days were inserted in the middle of summer between the 3rd and 4th months, the month following the epagomenal days would start on a Sunday (and so would the 5th month of Winter). If the leap week were appended to the epagomenal days, it too would begin on a Sunday.


I reckon months start as follows:



Month 1 Thursday 9-15 April

Month 2 Saturday 9-15 May

Month 3 Monday 8-14 June

Epagomenal days Wednesday 8-14 July

Month 4 SUNDAY 13-20 July

Month 5 Tuesday 12-19 August

Month 6 Thursday 11-18 September



Month 1 Saturday 11-18 October

Month 2 Monday 10-17 November

Month 3 Wednesday 10-17 December

Month 4 Friday 9-16 January

Month 5 Sunday 8-15 February

Month 6 Tuesday 10-16 March


Month 4 Summer to Month 5 Winter start on the last day of their range only if the February in those months has 29 days. Both the first and last month in the range start on Sunday.



"Mapping Time" said the months fell out of use and only the weeks were used. I expect this was because the months were replaced by Julian calendar months, while the weeks continued to be used like ISO week numbers to schedule certain events such as the Icelandic parliament.  This may have happened before or when the leap week rule was reformed so that Summer starts on the Thursday between 9 & 15 April and so the months as listed above may never have been used.