Cold Food Festival, held on April 4 if it is a leap year (China); and its related observances:

The Cold Food or Hanshi Festival (寒食节) is a traditional Chinese holiday which developed from the local commemoration of the death of the Jin nobleman Jie Zitui in the 7th century BC under the Zhou dynasty, into an occasion across East Asia for the commemoration and veneration of ancestors by the 7th-century Tang dynasty. Its name derives from the tradition of avoiding the lighting of any kind of fire, even for the preparation of food. This practice originally occurred at midwinter for as long as a month, but the hardship this involved led to repeated attempts to ban its observance out of concern for its practitioners. By the end of the Three Kingdoms period, it was limited to three days in the spring around the Qingming solar term. Under the Tang, ancestral observance was limited to the single day which is now the Tomb-Sweeping Festival. The Tomb-Sweeping Festival is an official holiday in several countries, and the Cold Food Festival which stretches either side of it continues to see some observance in China, South Korea, and Vietnam.

A leap year is a calendar year that contains an additional day compared to a common year. The 366th day is added to keep the calendar year synchronised with the astronomical year or seasonal year. Since astronomical events and seasons do not repeat in a whole number of days, calendars having a constant number of days each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("intercalating") an additional day—a leap day—or month—a leap month—into some years, the drift between a civilisation's dating system and the physical properties of the Solar System can be corrected.