Samhain falls precisely between the Autumn Equinox and the Winter Solstice.
And as this site says, “do you really think that the ancient Brits spent a thousand years or so building and perfecting Stonehenge so that they could isolate the exact moment of the summer solstice, only then to decide ‘hey! let’s hold the celebration on Monday instead so we can have a three day holiday?’”
For this reason I’m making this thread:
1: to raise awareness this is even a thing, in case it assists anyone - I didn’t know it when I was younger;
2: for the benefit of anyone who wants to take part in this group working and do so on Samhain;
3: because maths and numbers are cool and fun! 
So, all times are UTC because that keeps things simple:
Autumn Equinox falls on Septermber 22nd at 20:02 UTC
Winter Solstice falls on December 21st at 16:28 UTC
I then did a LOT of pencil & paper maths. Got it wrong first time round! 
Will spare you that.
TimeAndDate, my one and only website crush, affirms my final figure though - it calculates the gap between those times on those dates to be 129,266 minutes.
When I input the same start date and my proposed time (2nd try) for the halfway figure, it states that it’s 64,633m - exactly halfway!
Phew.
So, looks like the Samhain our stone-circle building ancestors laboured for, and I did a few mouseclicks to get (coz evolution facilitates stupidity) is:
This will be 14:15 (2.15pm_) EST - I gave the godlike page NYC as an example, so I can give you this link to generate the time in your own timezone:
Time & Date Timezone Converter, Samhain Settings
Select “Make changes to the current search” - the first field is the time UTC, the second you need to adjust to your own timezone, so it can tell you what hour that falls in.
I’ll take further comments on that proposed working to its own thread, but I thought a tour round my OCD might be nice for y’all, of a Monday!
