Sun Samhain (pics below of 2020, 2021 and 2022) is when the Sun sets with the heliacal setting of one of the 4 royal stars called Antares in Scorpio, we call this star Tlachtga. Tlachtga sets below the western horizon shortly after sunset. This is the last visible setting of the star until next year. This is the "heliacal setting" when the star is at 3* above the horizon and sets after sunset when the Sun is already 7* below ie far enough below the horizon to let the star become visible just before setting. The angle of the star to the horizon enlarges it to make it seem bigger but only for a few seconds - a wink if you like.
Interpolation is a mathematical technique used by those who want to escape the commercial church calendar but have not enough knowledge or awareness of the obvious links of the Eightfold Year to the Zodiac and its four bright stars. This is far too detailed for fb but is a part of our online course at https://www.celticdruidtemple.com/druidschool.html
This year Sun Samhain is on the 2nd of November. Watch out for the bandits who use the catholic calendar and call halloween 'Samhain' and for the interpolators who say Samhain is when the Sun is at 15* of Scorpio because they calculated it with a slide rule and a catholic calendar... Note - there is no visible star at 15* of Scorpio while one of the 4 brightest stars of the Zodiac shines very brightly but briefly on the 2nd of November just after the Sun has set! Do you reckon our ancestors used slide rules, mathematics and calendars that did not exist back then to time their celebrations for the Eightfold Year or do you reckon they looked to the Realm of Sky???