The ketuba is a shtar, and official document, and therefore it must be properly dated. What if the wedding, i.e., the huppa, is meant to take place at night, but the hatan performs a kinyan on the ketuba during the day? In this case, although the huppa is at night, the kinyan is in the day. Dating the shtar for the date of the night would be a shtar meuhar, a post dated shtar. Meaning it is dated later than when the transaction actually happened. Preferably this should not be done.
Rather, the Ben Ish Hai (Rav Pealim 3:3) and Sefer Nehar Mitzraim (Hilkhot Ketuba) record that the Sephardic custom was to date the ketuba during the day, and perform the kinyan during the day as well, even though the wedding was held at night. Since the ketuba is merely a financial obligation, and is not dependent upon the huppa, the ketuba is valid. Although R. Moshe Feinstein (Iggerot Moshe, EH 4:100) did not approve of this practice, this is indeed the common custom, as recorded by the Ben sIsh Hai and Nehar Mitzrayim.
Some claim that since the witnesses sign the ketuba which describes the kiddushin, if they sign the ketuba the day before the wedding, the ketuba appears to be forged. However, if this were to be true, almost all ketubot would be invalid, as it is customary to sign the ketuba before the wedding, not after the kiddushin. Rather, the witnesses are signing a document with the understanding that this is would will happen immanently. However, this does not invalidate the ketuba.
Summary: When a wedding is held at night, and the ketuba is signed and a kinyan is performed during the day, it is best that the date on the ketuba reflect the kinyan (i.e., the day), and not the huppa (night).
