I could find no International Holiday for December 14, but seems to be quite the day for birds. First of all, it is the beginning of what are known as Halcyon Days, a period of calm and relative warmth during the cold of winter.
Its origin is from Roman mythology, when Alcyone (or Halcyon) daughter of the wind god Aeolus, fell in love with a mortal sailor, Ceyx. Ceyx was drowned during a storm, and Alcyon dove in after him and disappeared. The gods were so touched they decided to transform the two of them into birds so they could always be together in flight above the rough seas. Aeolus also agreed to maintain the seas calm for a two-week period each year, enabling Halcyone and Ceyx to nest on the water and lay eggs.
Halycon Days are 14 days around the December Solstice and generally thought to commence on December 14.
December 14 will also kick off Audubon’s 120th Christmas Bird Count, which will last from Saturday, December 14, 2019 until Sunday, January 5, 2020.