dead of night
The middle of the night. Why are you calling me in the dead of night? Can't this wait till morning?
dead of winter
The middle of winter, which is usually especially cold. I find myself dreaming of tropical islands every year in the dead of winter.
The period of greatest intensity of something, such as darkness or cold. For example, I love looking at seed catalogs in the dead of winter, when it's below zero outside. The earliest recorded use of dead of night, for "darkest time of night," was in Edward Hall's Chronicle of 1548: "In the dead of the night ... he broke up his camp and fled." Dead of winter, for the coldest part of winter, dates from the early 1600s.