Is the marriage contract valid if fornication occurred before the marriage, then the wife experienced her menstrual period on the morning of the contract day and the contract was concluded in the afternoon, taking into consideration what is known as the waiting period for clearing the womb (iddat al-istibra’)?

You must sincerely repent from adultery, abandoning it, regretting it, and resolving never to return to it. Your marriage to the Christian woman is invalid because she is not chaste, and a condition for marrying a woman of the People of the Book is that she be chaste. Al-Sa'di said: "Immoral women who are not chaste from adultery, it is not permissible to marry them, whether they are Muslim or from the People of the Book, until they repent; due to His, the Almighty's, saying: {The adulterer does not marry except an adulteress or a polytheistic woman}." You must separate from her, and it is not permissible to marry her unless she repents. It is preferable for a Muslim to marry a righteous Muslim woman.