Matsu (Mazu) is a sea goddess in the Taoist tradition, and she is the protector of sailors, fishermen and all people whose lives are connected with the sea. As a result, the worship of this goddess is widespread in countries like Taiwan and Vietnam, where so many people live by the sea. Her official worship in China dates back to the 12th century.
The date for this festival moves from year to year in the Gregorian calendar because it is based on the Chinese calendar, which is a lunar calendar. The festival takes place on the twenty-third day of the third month, the "peach-blossom" month.
According to some legends, the goddess Matsu was originally a mortal woman, named Lim Bek-niu, who lived in the tenth century. She came from a family of fishermen. One day when her father and brothers were out fishing, a typhoon blew up. Lim Bek-niu began to pray for her father and brothers and fell into a trance. She saw her father and brothers and was able to reach out to them and hold them up in safety from the water, but when her mother tried to wake her from the trance, she dropped her father and he died at sea, so that only her brothers returned home safely. She then swam far into the ocean in order to try to rescue her father, and her body finally washed ashore on one of the Matsu Islands, a set of islands in the Taiwan Straits. In other versions of the story, she did not die at sea, but instead she climbed a mountain where she was lifted up into the heavens and transformed into a goddess.
After her death, people began to revere her memory and her worship as a goddess spread throughout Asia and has continued until modern times. Chinese and Vietnamese immigrants to the United States have built Matsu templates in cities such as San Francisco and Los Angeles.
You can view a video online about Matsu worship in Taiwan at the Republic of China on Taiwan Multimedia Gallery.
Here is an image of a Matsu statue at Chua Ba Thien Hau (Camau Association of America) in Los Angeles: