Although, goddess Sita's birth as per most scriptures, specifically the original Valmiki Ramayan is said to be non-uterine (ayonija), still the day of her being discovered by king Janaka maybe considered as her birth-date (tithi).
This is how the Valmiki Ramayan describes it -
Janaka describes Sita's birth as non-uterine.
भूतलादुत्थिता सा तु व्यवर्धत ममात्मजा । वीर्यशुल्केति मे कन्या स्थापितेयमयोनिजा ॥ १-६६-१४ ॥
- Hers is a non-uterine birth as she surfaced from the surface of the earth, but fostered as my own soul-born girl and I determined [to give her in marriage to a bridegroom where his] boldness is the only bounty, I receive in that marriage.
तस्य लान्गल हस्तस्य कर्षतः क्षेत्र मण्डलम् । अहम् किल उत्थिता भित्त्वा जगतीम् नृपतेः सुता ॥ २-११८-२८ ॥
- "While he was furrowing a territory of land, holding a plough in his hand, it is so said that I came forth, splitting up the land, as a daughter of that king."
अन्तरिक्षे च वाग् उक्ता अप्रतिमा मानुषी किल । एवम् एतन् नर पते धर्मेण तनया तव ॥ २-११८-३१ ॥
- "It is said that voice resembling that of a human being in the air above me rang out, saying": "O, King! Let it be so. This divine child without a match is a daughter to you, rightly."
However, nowhere, I can locate a date (tithi) on which Sita was "born" or "discovered" by Janaka, unlike Sri Rama for whom Sri Valmiki gives an exact tithi in V.R. 1.18.8,8,10,11, popularly known as Rama-Navami ( celebrated on Chaitra Shukla Paksha Ninth).
Interestingly several articles give a date of birth (tithi) for Sita too, as Vaishakha Shukla Paksha Navami as Sita Jayanti or Sita Navami.
This is what the famous vedic jyotish website Drikpanchanga writes -
Sita Navami is celebrated as birth anniversary of Goddess Sita. This day is also known as Sita Jayanti. Married women keep fast on Sita Navami day and pray to seek long lives of their husbands.
Sita Jayanti is celebrated on Navami Tithi during Shukla Paksha of Vaishakha month. It is believed that Goddess Sita was born on Tuesday in Pushya Nakshatra. Goddess Sita was married to Lord Rama who was also born on Navami Tithi during Shukla Paksha of Chaitra month. On Hindu calendar Sita Jayanti falls after one month of Rama Navami.
Are there any scriptural references for goddess Sita's birth tithi, Sita Jayanti?
What scriptures (or sects) describe or believe the birth or discovery of goddess Sita on the month of Vaishakha in the Shukla Paksha on the Navami tithi?