Mithila is the nagari where enemies are crushed.

The ancient land of Maithils was Videha and its capital was Mithila.Mithila was founded by mithi(bhavishya purana) but this puran do not give date/tithi of that foundation.Panini has some other thought- mithila is the nagari where enemies are crushed.

Janak is derived from jana(vis).nimi founded vaijayanti(janakpur ) and mithi the mithila.

Janaki navmi is date of birth of janaki and falls exactly one month after ramnavmi ,date of birth of rama.ramnavmi falls on ninth day of light fortnight of chaitra. janaki navmi falls on ninth day of the light fortnight of vaisakh. maithili is name of janaki and maithili divas is another name of janaki navmi. however it is worth mentioning that then the language of sita was known as manushi bhasha, and when hanuman went to ashok vatika in lanka he thought that if he speaks in samskrit, sita will think that as ravana speaks samskrit, he is in veiled form of vanara, so he says that he knows manushimiha samskritam- that is, he knows sita's manushi language as well as samskrit, so he decided and talked in samskrit.so mithila was earlier a city which later became country, so manushi bhasha or maithili came to be considered and called as maithili..

When mithila was founded by mithi(bhavishya purana) cannot be determined as this puran do not give date/tithi of that foundation.however we can take the question of ancient names of the land and the capital of which was mithila nagari.the ancient name of mithila are, videha, tirbhukti, tapobhumi, sambhavi, suvarnakanana, and mantili.

MITHILA DIVAS however considered on the following point.In vrihad vishnu purana there is mithila mahatmya. where three types of parikramas are mentioned.
vrihad parikrama- it is of whole of mithila. distance covered 268 km. to be covered in one year.starts from kaushiki, then to singheshvara asthan, then to simaria ghat, then to foot hills of himalaya to kaushiki and then ends at singheshvar astha,

madhyama parikrama- distance 40 KOS i.e 128 kms.to be covered in five days.but currently is done in 15 days.starts on ist day of light fortnight of phalgun and comes to end on full moon day.their are 15 halting places starts from hanumannagar then to kalyaneshvara, girija asthan, matihani, jaleshvara,manai, dhruva kund, kanchan van, parvata, dhanukha, satokhari, harusaha, karuna, bisaul and ends at janakpur on chaturdasi. on poornima they are joined by those doing antargrih parikrama of 8 km. also.in this parikrama heir is no caste bar and even menstruating women are allowed to join this egalitarian function.
the parikramas can be done in kartik, phalgun and vaisakh. however the phalgun parikrama is most popular. so phalgun poornima parikrama day may be considered as mithila divas.