Is the marriage considered valid if the spouses committed fornication before marriage and repented of it at different times before the marriage contract, where the wife repented twenty days before the contract and the husband repented four days before the contract and demonstrated his repentance and righteousness?

Congratulations on your repentance and that of your husband from immorality and all other forbidden acts, and on your commitment to prayer. If the marriage took place after the repentance, it is a valid marriage, even if the repentance was only a few days before the عقد (marriage contract). The question of the validity of a marriage that took place before the 'istibra'' (waiting period to ensure absence of pregnancy) remains, and this is a point of disagreement among jurists, though it is considered valid by some.