I saw this phrase on Instagram regarding a winter garden and had to look it up. According to this master gardener column,

Elliot Coleman, author of many books on organic gardening, uses the term “Persephone period” or “Persephone days” to describe the time of year when the days have 10 or fewer hours of light.

After about January 25th in our latitude, we start getting a little more than 10 hours of light–helpful if you’re planning spring seeds or just so, so over the dark and cold.