The dark winter they refer to is the winter solstice for us. December 21-23. Throughout occultism these are considered the days of the dead due to the sun being still and not moving. Here is a good article that sums it up...

Some dates they utilize for occult cermonies:

The occult calendar is comprised of four periods of 13 weeks relating to the sun solstices and Equinox: Winter Solstice Period extends 13 weeks — Minor sabbath

December 21 — Yule
December 21-22 — Winter Solstice/Yule. One of the Illuminati’s Human Sacrifice Nights
December 25th — Holy Roman Christmas
February 1 and 2 — Candlemas and Imbolg, a.k.a. Groundhog’s Day. One of the Illuminati’s Human Sacrifice Nights
February 14 — Valentine’s Day

Roman Saturnalia: The Romans celebrated Saturnalia, a festival of general merrymaking and debauchery held around the time of the winter solstice. This week-long party was held in distinction of the god Saturn and involved sacrifices, gift-giving, special privileges for slaves, and much feasting. Although this holiday was partly about giving presents, more important, it was to honor an agricultural god. On the heels of the Saturnalia, the Romans marked December 25 with a celebration called the Brumalia. Bruma is thought to have been contracted from the Latin brevum or brevis, meaning brief or short, denoting the shortest day of the year.

https://stillnessinthestorm.com/2015/12/occult-meanings-of-winter-solstice-and/