Sorry David, i do have zany humour, i don't mean anything by it, just a mad English man!
Hi Opal,
I thought the date should go back to the beginning of Stonehenge, i know modern druids celebrate the Summer Solstice on 21st June, however all the evidence points to ancient people valuing the Winter Solstice instead.
It makes sense as it is the shortest day when the Sun begins to return, and the days get longer.
Places like the tomb of Maeshowe in Scotland show this with the setting Sun entering the tomb on 20th December being ancient Greek day marker for the Winter Solstice on 21st December, video below:-
https://www.youtube.com/watch?v=uPNII19Qn4Y
Link to Maeshowe below:-
https://en.wikipedia.org/wiki/Maeshowe
Now if you read about the Julian Calendar on link below, where my avatar is Archaeoastronomer, you will understand that Julius Caesar made 25th December the Winter Solstice in 0045BC.
https://grahamhancock.com/phorum/read.php?5,1264020,1264709#msg-1264709
I can go backwards in time from there by using a Proleptic Julian Calendar:-
https://en.wikipedia.org/wiki/Proleptic_Julian_calendar
However the Julian Calendar has errors, so the Winter Solstice will slip backwards through the Calendar and researchers think that Stonehenge was starting to be built approx. 3100BC:-
https://en.wikipedia.org/wiki/Stonehenge
Stonehenge is near the town of Amesbury, Wiltshire, so am using this for a fix for Lat. and Long.:-
https://en.wikipedia.org/wiki/Amesbury
Just after midnight on the Winter Solstice in 3100BC there was an alignment with Jupiter and Sirius that were culminating, but have no idea of what ancient Britons believed about Jupiter or Sirius so long ago but having the two aligned, would have been spooky for them as a sign in the sky, whether or not it was a sign to build, i can't say!
I have tried to calculate where the Winter Solstice would fall in 3100BC, i think i have it right as 28th November, but would work for the day before or after being 14 Minutes after midnight, i hope you find it interesting, graph below:-