Maharaja Janaka adopted the girl as his own daughter. This is the reason that Sita is also called as Janaki- the daughter of Janaka.
As this day happens to be on a navami tithi, it is also called as Sita Navami or Janaki Navami.
Goddess Mata (mother) Sita is revered by many in India. She is lovingly referred to as Mata Sita as well as Sita Maiya, which is another way to address her as mother Sita.
She is particularly revered by married women and many times; married women keep a fast on Sita Navami (Janaki Navami) day to obtain the Goddess’s blessings and seek long lives for their husbands.