Candlemas, a holiday that originated in the 4th century, was celebrated 40 days after Christmas (or Epiphany) - on February 2nd. On this day were big feasts and the church candles were blessed for the coming year (thus the name, Candlemas). People burned holiday greenery (leftover yule logs, etc) so the ashes could be used as fertilizer for the prepping of the fields. The burning of greenery and candles was symbolic of the coming light of Spring and regrowth/rebirth. The holiday also marked the start of the plowing of fields.
According to Churchyear.net,
"Thus if the sun cast a shadow on Candlemas day, more winter was on the way; if there was no shadow, winter was thought to be ending soon. This practice led to the folklore behind "Groundhog's Day," which falls on Candlemas Day."
For more info:
How Candlemas was celebrated in the Middle Ages
When Christians disposed of trees and holiday greenery