mid-winter; cold season
Refers to the coldest part of winter, roughly from early January to early February. Often used in seasonal greetings, weather reports, and expressions like 寒中見舞い.
The period for sending mid-winter greeting cards is from after the New Year's pine decorations are taken down until the beginning of spring.
It gets especially cold during the mid-winter season, so be careful about your health.
In this coldest season, I hope you are doing well.
I take a walk every morning during the coldest part of winter.