Mother-In-Law Day is always the fourth Sunday in October. Its observance was started in 2002 to celebrate those special (and not so special) In-Laws! The source of many jokes, a mother-in-law doesn't usually get the praise and appreciation she deserves. According to a poll by iVillage, 72 percent of people rated their relationship with their mothers-in-law as fair to excellent, with only 28 saying it was "bad" or "terrible."