Constructing the Ingwine Calendar

When creating the Ingwine calendar, the primary source I look to is the Venerable Bede. In De Temporum Ratione, Bede provides the following pieces of information, from which I reconstruct the Ingwine calendar:

• New days begin at sundown
• A new lunar month begins when the new crescent moon is visible to the naked eye prior to sundown. This is a bare minimum of 15.5 hours after the astronomical new moon.
• The astronomical winter solstice occurs in the first Yule month.
• In a typical year, the summer solstice occurs in the first Litha month.
• In a three Litha year, the summer solstice occurs in the middle Litha month.

In order to calculate the dates for the calendar, you need three pieces of information: the time of sunset, the time of the astronomical new moon, and the time of the winter solstice. I arbitrarily chose London as the locale for these times for the northern hemisphere calendar and I use Amsterdam as my secondary check for edge cases. It turns out that the calculations are surprisingly consistent for nearby locations, even in the edgiest of edge cases (such as 2025). If you use a locale further away from GMT, there will be inconsistencies in the calendar. I have found that for 2025, my calculations hold until about GMT-5. After that, you would have the new moon visible on the evening before the astronomical winter solstice, which would shift all of our months. This is why there are inconsistencies with the calculations of the 2025 and 2026 calendars that other people have created.

To illustrate, my rationale for the calculation of the Ingwine calendar for 2025-2026 follows. All times are in GMT.

• The December astronomical new moon occurs at 01:43 on December 20th. 
• Sunset occurs at 15:13 on December 20th. This is a difference of 14 hours and 10 minutes. Therefore, the new moon will not be visible prior to sunset on December 20th and the new lunar month will begin at sundown on December 21st. 
• The Winter solstice occurs at 15:03 on December 21.
• Sundown on December 21 occurs at 15:53.
• The month that begins at sundown on December 21st is beginning 50 minutes after the astronomical winter solstice, therefore that month is the Æfterra Gēola month.

Æfterra Gēola – begins at sundown on December 21 2025. 
Æfterra Gēola full moon – January 3, 2026 at 10:02.
Astronomical new moon: January 18, 2026 at 19:51. This is after sunset, so Sol-mōnaþ will begin at sundown on January 19 2026.

Sol-mōnaþ – begins at sundown on January 19, 2026.
Sol-mōnaþ full moon – February 1st at 22:09
Astronomical new moon – February 17th, 2026 at 12:01. Sundown is at 17:19, so Hrēþ-mōnaþ will begin at sundown on February 18.

Hrēþmōnaþ – begins at sundown on February 18, 2026.
Hrēþmōnaþ full moon – March 3, 2026 at 11:37.
Astronomical new moon – March 19 2026 at 01:23. Sundown is at 18:11. This is a difference of 16 hours and 48 minutes. The crescent moon should be visible prior to sundown (assuming clear weather), so Easter-mōnaþ begins this evening.

Eastermōnaþ – begins at sundown on March 19, 2026.
Eastermōnaþ full moon – April 2, 2026 at 01:11.
Astronomical new moon – April 17, 2026 at 11:51. Sundown is at 19:00. This is a difference of less than 8 hours, therefore the new month cannot begin until the evening of April 18.

Þrimilcemōnaþ – begins at sundown on April 18, 2026.
Þrimilcemōnaþ full moon – May 1, 2026 at 17:23. 
Astronomical new moon – May 16, 2026 at 20:01. This is after sundown, so Ærra Līþa begins on the evening of May 17.

Ærra Līþa – begins at sundown on May 17, 2026.
Ærra Līþa full moon – May 31, 2026 at 8:35.
Astronomical new moon – June 15, 2026 at 02:54. Sundown is at 20:19. This is a difference of 17 hours and 25 minutes, so the new moon should be visible prior to sundown.

Līþa – Begins at sundown on June 15, 2026
Summer Solstice – June 21, 2026 at 08:24.
Līþa full moon – June 30, 2026 at 11:56.
Astronomical new moon – July 14, 2026 at 09:43. Sundown is at 20:12. Therefore, Æftera Līþa will begin at sundown on July 15, 2026.

Æftera Līþa – begins at sundown on July 15, 2026
Æftera Līþa full moon – July 29, 2026 at 14:53.
Astronomical new moon – August 12, 2026 at 17:36. Therefore, Wēod-mōnaþ will begin at sundown on August 13th. 

Wēodmōnaþ – begins at sundown on August 13, 2026.
Wēodmōnaþ full moon – August 28, 2026 at 04:18.
Astronomical new moon – September 11, 2026 at 03:26. Sundown is at 18:24. That is just less than 15 hours, so the new moon cannot be visible prior to sundown and the next month will begin at sundown on September 12.

Hāligmōnaþ – begins at sundown on September 12, 2026.
Hāligmōnaþ full moon – September 26, 2026 at 16:49.
Astronomical new moon – October 10, 2026 at 15:50. Sunset is at 17:18, therefore the next month will begin at sundown on October 11.

Winterfylleth – begins at sundown on October 11, 2026.
Winterfylleth full moon – October 26, 2026 at 04:11.
Astronomical new moon – November 9, 2026 at 07:02. Sundown is at 16:19. This is a difference of just over 9 hours, so the new moon cannot be visible and the next month will begin at sundown on November 10.

Blōtmōnaþ – begins at sundown on November 10, 2026.
Blōtmōnaþ full moon – November 24, 2026 at 14:53.
Astronomical new moon – December 9, 2026 at 00:51. Sundown is at 15:51, which is less than 15.5 hours, so the next month will begin at sundown on December 10.

Ærra Gēola – begins at sundown on December 10, 2026.
Ærra Gēola full moon – December 24, 2026 at 01:28.
Astronomical new moon – January 7, 2027 at 20:24. This is after sunset, so the next month will begin at sunset on January 8, 2027.