寒中見舞い (kanchuumimai, midwinter greetings) or 寒中お見舞い (kanchuu o-mimai) are postcards that people in Japan send to friends during mid winter. They typically contain the phrase "寒中(お)見舞い申し上げます" (kanchuu (o-)mimai moushi agemasu).
Unlike shochuumimai, kanchuumimai are often used as substitutes of nengajou. When you were not able to send nengajou during New Year holidays for some reason, or when you received 喪中欠礼 (mochuu ketsurei) from one of your friends, they will be sent. (Mochuu ketsurei means "Please forgive me not sending you New Year greetings. Obeying Japanese custom, I shall refrain from offering you New Year's greetings while in mourning.")
